List grouping key?

What does “list grouping key” at the end of a showme item command mean?

When the game is listing things, you can tell it to group certain ones together. For example, “You are carrying an X scrabble tile, a Z scrabble tile, a Q scrabble tile, and a J scrabble tile” is a lot more annoying to read than “You are carrying four scrabble tiles (X, Z, Q, and J)”.

The “list grouping key” is used to make this happen. Normally you don’t set it directly, you use the “group [items] together as [text]” phrase.