A problem-type is a collection of files inside <GiDdirectory>/problemtypes
that customize GiD for a particular type of simulation. When a problem-type is loaded it can modify the whole GUI: menus, toolbars, windows, etc., and can, for example, add a menu that calls the Tcl code that executes the Python command of interest.
A plug-in of GiD is simply a collection of files inside <GiDdirectory>/plugins
that are loaded when GiD starts. They can modify the GUI, for example adding a menu entry to invoke some new feature. Like previously, the Tcl code can run Python code (e.g. Real example: meshio GiD plugin
)
Note: the features added by plugins are available without loading a problemtype, and are preserved when changing to another problemtype.