Presentations can overlap on the screen, so there can be more than one presentation at the pointer location. Often when two presentations overlap, one is nested inside the other.
One cause of nesting is presentations of compound objects. For example, a sequence of pathnames has one presentation for the sequence, and another for each pathname.
When there is more than one candidate presentation at the pointer location, CLIM must decide which presentation is the sensitive one. It starts with the innermost presentation at the pointer location and works outwards through levels of nesting until a sensitive presentation is discovered. This is the innermost presentation that has any applicable presentation translators to any of the nested input context types for the currently pressed chord of modifier keys. Searching in this way ensures that a more specific presentation is sensitive. Note that nested input contexts are searched first, before nested presentations. For presentations that overlap, the most recently presented is searched first.
Common Lisp Interface Manager 2.0 User's Guide - 20 Sep 2011