The aecomp script is used to compare two changes, using tkdiff(1) to display the changes. If you give one change on the command line, the other change is determined in the usual way. If you give two changes, those are the two compared. Both changes must be in the being developed state (it's only a script, after all).
Basically, aecomp allows you to specify two project/change pairs (ie the compared changes don't have to be in the same branch or project). aecomp attempts to use defaults (for project and change number) where possible. As a minimum, it needs a single change number as an option. A list of files which are common between the two changes is constructed and presented. Double clicking on any of the file names will tkdiff the two versions (ie the files in each change).
It is useful after you have used aeclone(1), then modified a change and subsequently are wondering what on earth you did. Files are considered to be "common" if they have the same name. In the case of a file which has been moved, it's original filename is used (the diff of course takes place against the new file name).