On the QL we have traditionally had a 36 character limit on file names after the initial (5 character) device name. Something like this:
flp1_source_code_myfile_c
In the above, flp1_ is the device name and is not included in the 36 character limit. There can be a network device tagged on the front as well, and this too is not included in the filename limit.
Technically, in the above, source and code are both directories (the default directory separator is the underscore character) while myfile_c is the filename and extension.