Sometimes the file name can clue you in to how it is being used. For example, if it is being used as a diagnostic file as OkiePC mentioned, then the file would be automatically named "CH0 DIAG" or "CH1 DIAG". The Usage for these would all show as unused, but the name tells me their purpose.
As for the Usage function, if you see a letter then that indicates it is in use. You might see "F" in the FW column if the addresses are used by a file type instruction (like a Bit Shift). You might see an FW if a word within that file is being used. Or you might see "X" if it is used by a Word or Bit type instruction. The "F" and "FW" are mainly seen when viewing usage with binary files.
And of course, it is entirely possible that the file is not being used. Maybe it was being used at one point, but they deleted the code and the data remained.
OG