A ".plt" file is a platform-independent distribution archive for software based on Racket. A typical ".plt" file can be installed as a package using raco pkg (see Package Management in Racket), in which case raco pkg supplies facilities for uninstalling the package and managing dependencies.
An older approach is to supply a ".plt" file to raco setup with the -A flag; the files contained in the ".plt" archive are unpacked (according to specifications embedded in the ".plt" file) and only collections specified by the ".plt" file are compiled and setup. Archives processed in this way can include arbitrary code that is executed at install time, in addition to any actions triggered by the normal collection-setup part of raco setup.
Finally, the raco unpack (see raco unpack: Unpacking Library Collections) command can list the content of a ".plt" archive or unpack the archive without installing it as a package or collection.