Since the file name ends with '.scm', it is a script, so it goes in the scripts subdirectory (if you put it in plug-ins I wonder what Gimp is executing...). If you go to Edit>Preferences>Folders>Scripts the right pane will list two directories, one is R/W (in your own user directories) and one is R/O (in the Gimp installation).
If you use the "private" directory:
So in general, it is best to use the "private" location.
This is also true of other additions: brushes, patterns, gradients, and at least the simpler plug-ins (Python ones).
If you use the "private" directory:
- you can add things there without requiring admin rights
- if you share the machine with other people, they won't be impacted by your addition
- it is more likely to be part of your regular backups
- it won't be lost if you re-install Gimp
- you can share it with other users of the machine
So in general, it is best to use the "private" location.
This is also true of other additions: brushes, patterns, gradients, and at least the simpler plug-ins (Python ones).