Since variations of this question have popped up many times over the years, I will try to explain step-by-step what’s happening when you get the unexpected behavior, i.e. when trying to take the slices even though the player is already carrying them indirectly as the contents of the delivery box.
THE OUT-OF-THE-BOX EXPERIENCE
The grammar lines for the word “take” can be seen using the debugging command >SHOWVERB TAKE, which produces:
Verb 'carry' 'hold' 'take'
* 'inventory' -> Inv
* multi -> Take
* 'off' noun -> Disrobe
* noun 'off' -> Disrobe
* multiinside 'from' noun -> Remove
* multiinside 'off' noun -> Remove
These all come from the Standard Rules. The output is in I6 style, so you need to do some translating to make sense of it.
When all of the pizza is enclosed by the player and the entered command is >TAKE SLICES, the only grammar lines that matter here are the second, fifth and sixth. The second uses a “multi” token, which comes from the I7 grammar line:
Understand "take [things]" as taking.
The fifth and sixth use a “multiinside” token. They come from the I7 grammar lines:
Understand "take [things inside] from [something]" as removing it from.
Understand "take [things inside] off [something]" as removing it from.
When evaluating the second grammar line, the parser sees the word “slices” in the place where it will be looking for words that match objects. It will correctly identify all pizza slices in scope as matching that word. However, it will also check whether anything being put on the multiple object list belongs there according to the deciding whether all includes rules.
As you’ve seen, the exclude indirect possessions from take all rule is a rule in that rulebook, defined in the Standard Rules as:
Rule for deciding whether all includes things enclosed by the person reaching
while taking or taking off or removing (this is the exclude indirect
possessions from take all rule): it does not.
Since every pizza slice is enclosed by the person reaching (where person reaching is another way of specifying actor, used here because this rule is checked outside of action processing), and since the action being parsed is a taking action, every pizza slice is then kept off the multiple object list, leaving it empty.
Coming back from the deciding whether all includes rules, the parser sees that there is nothing left to be a noun and fails the grammar line. However, it’s not done trying grammar lines for the word “take” and moves on. The next two fail, and it continues to the fifth line.
Even though the command that was entered is only >TAKE SLICES, the parser is trying to match a line of the form >TAKE [THINGS] FROM [SOMETHING CONTAINING THOSE THINGS]. Part of what it does for this type of line is a “second noun lookahead” to try figure out what the second noun will be, so as to be able to know whether the nouns are inside of it. Another part of what it does is “inference,” i.e. to try to guess what the player meant when it thinks part of the command is missing.
Here, those two bits of logic interact to make the parser temporarily assume that the player really meant >TAKE SLICES FROM [SOMETHING TO BE DETERMINED]. It then attempts to make a best guess about what that undetermined thing is according to some fairly generic rules. These rules make the parser assume that, because the verb is removing it from, something which is in the location (i.e. “on the floor” of or “directly in” the current room) is more likely to be the correct object.
In your scenario, the only such object is the closet door, so it selects this as the best guess. The parser’s working assumption at this point is that the “real” command intended by the player is >TAKE SLICES FROM CLOSET DOOR, and it helpfully informs you that That can't contain things., where “that” means the closet door.
If the player carries every pizza slice and the command is >TAKE SLICES FROM BOX, then the parser will look at the second grammar line first, as before, and reject every slice due to the exclude indirect possessions... rule, as before. (Arguably these are not “indirect” possessions, but the actual logic uses the enclosure relation, so directly carried things still count.)
When it moves on to the fifth grammar line, however, the second noun lookahead will see that the word “from” appears later in the command input and try to resolve the words following it as an object. It has no trouble matching the word “box” to the delivery box, so it takes note of that object and stores it in an I6 global variable called advance_warning.
It then rewinds to look at the words between “take” and “from” to see if they can be matched, but, because advance_warning is defined as the delivery box, the parser is now checking to make sure that any matching item is in the delivery box. All of the pizza meets this condition, but it is also enclosed by the player, so it is once again excluded by the exclude indirect possessions... rule and the multiple object list ends up empty.
This is a slightly different error state, in that the delivery box was correctly identified as the second noun, and it is a container. The resulting default message is There are none at all available! because the parser didn’t see any of the contents of the box as part of what is “included by all” (i.e. allowed on the multiple object list).
That’s a lot of complexity emerging from what looks like a simple situation. To review, this is the default behavior:
Parlor
You can see a closet door here.
>test me
(Testing.)
>[1] open box
You open the delivery box, revealing five pizza slices.
>[2] take slices
That can't contain things. [parser error]
>[3] take slices
That can't contain things. [same state, same parser error]
>[4] put slices in box
There are none at all available!
>[5] take slices from box
There are none at all available! [parser error]
>[6] take slices from box
There are none at all available! [same state, same parser error]
Compare that with what happens when the player does not hold the pizza box.
Parlor
You can see a closet door here.
>test me
(Testing.)
>[1] drop box
Dropped.
>[2] open box
You open the delivery box, revealing five pizza slices.
>[3] take slices [exclude indirect possessions rule doesn't apply]
pizza slice: Taken.
pizza slice: Taken.
pizza slice: Taken.
pizza slice: Taken.
pizza slice: Taken.
>[4] take slices [parser can't infer a second noun]
What do you want to take those things from?
>[5] put slices in box
pizza slice: Done.
pizza slice: Done.
pizza slice: Done.
pizza slice: Done.
pizza slice: Done.
>[6] take slices from box [exclude indirect possessions rule doesn't apply]
pizza slice: Taken.
pizza slice: Taken.
pizza slice: Taken.
pizza slice: Taken.
pizza slice: Taken.
>[7] take slices from box
You can't see any such thing. [parser error]
In the second situation, the first >TAKE SLICES or >TAKE SLICES FROM BOX works as expected because the exclude indirect possessions... rule does not apply; the slices are not enclosed by the player.
The second >TAKE SLICES (command 4) has a new message. That’s because when the parser gets to the inference stage while processing the fifth grammar line there are two non-carried things in the area this time (delivery box and closet door), and the parser can’t decide on a best guess based on default scoring. Accordingly, it assumes the word “from” and just asks for clarification about which item the slices should be taken from. If the player supplies that input as >BOX, then the command becomes >TAKE SLICES FROM BOX, and it works.
The second >TAKE SLICES FROM BOX (command 7) has a default can't see any such thing parser error because, while processing the fifth grammar line, it identifies the delivery box as the advance_warning, but there is nothing that matches the word “slices” that is inside the box. (I think but am not certain that in this case the parser never gets as far as considering the words “slices” to have matched any object, so it reports the error as though the command had been >TAKE LSDJFLSJD or other gibberish.)