I drew up this summary of what’s in the Index (under Actions->Tokens) and what’s in Chapter 17, which I find helpful as a reference. The hotlinks in the Index page are very useful for browsing.
An action needs two things to anchor it in the game world:
(i) (Photographing) is an action applying to nothing/(one/two) (visible/touchable/carried/–) (thing/value) [and one…] [and requiring light]. → a framework for how the action works in the game world
NB 1 thing here means object, so includes rooms and directions.
NB 2 confusingly, ‘visible’ here means ‘within the universe’ not the more usual ‘visible to the actor’. So it includes for example the compass directions and objects out-of-play- although by default objects not visible in the usual sense will be excluded from consideration by grammar lines (see below), usually with the retort ‘You can’t see any such thing.’. Without specifying ‘visible/carried’, ‘touchable’ is the default. ‘carried’ is a more restrictive form of ‘touchable’
(ii) Understand “photograph [someone]” as photographing. → grammar line(s) to recognise the action in what is typed
[somebody] / [someone] -> synonyms == a person in sight of the actor => the [second] noun (rather than 'the person understood')
[something] / [thing] -> synonyms == a thing in sight of the actor
[things] -> a list of one or more things in sight of the actor
[other things] -> a list of one or more things in sight of the actor which are not the other thing mentioned (e.g. a container)
[things inside] -> a list of one or more things in or on the other thing mentioned (e.g. a supporter/container)
[something preferably held] -> as with [thing], but choose a held object over others with same name, try implied take if present but not held
[things preferably held] -> as with [things], but choose a held object over others with same name, try implied take if present but not held
[text] -> an unparsed sequence of words => a snippet called 'the topic understood'**
[a time period] -> a period of time*** => a time called 'the time understood'
[description of objects/values] -> matching any description of objects ([open door]) within sight of the actor, or of values [even number]
-> [description of directions] references directions. NB directions are 'visible' but not 'touchable' (see above)
-> prefixing 'any' e.g. [any open door] removes the restriction that an object must be visible to the actor
-> [any thing] will allow any thing, in or out of the game world == [anything]; similarly [anybody], [anyone]
-> [any (description of rooms)] allows reference to rooms, not otherwise mentionable in typed commands
-> [anywhere] == [any room]
-> the Index indicates which built-in values have a corresponding Understand token
=> for values, a value called 'the (kind) understood'
=> for objects, the [second] noun
** this is something of an exception, for historical reasons, to the general ability to refer to a successfully-parsed grammar token as ‘the (kind) understood’, which here would otherwise be ‘the snippet understood’. Using ‘the noun’ and ‘the second noun’ to refer to the first and second object parsed are also exceptions. But we say for example ‘the number understood’ or ‘the time understood’ or ‘the colour understood’ for values. There is also ‘the person asked’ for constructions such as ‘Bob, open the box’.
*** this special token is provided because [a time], which follows the general [description of values] pattern, matches only specific times of day but does not match ‘21 minutes’ for example, even though this is also of kind time. However, the parsed value is still referred to as ‘the time understood’ rather than ‘the time period’ understood.