From IFWiki

An internal process of the parser library that is invoked whenever the player types a command that could refer to more than one object because two or more different objects have the same natural title.

For instance, if two hats are in scope -- one in the player's inventory and one on a hatrack -- and the player types "drop hat," the player is most likely referring to the hat in hand. If the parser is not able to determine which hat the player wants to interact with, the player will see a message such as, "Which hat do you mean, the tophat or the baseball cap?"