1. TTL flash metering (35mm) - let the camera meter work witin the range of the flash output to meter the flash light correctly.
2. Thyristor (on the flash unit) control - let the flash unit work within the range of the flash output to meter the flash light correctly.
3. Flash meter - use a flash meter to determine the appropriate aperture to set on the camera that will correctly expose film for the amount of flash light being used.
and there always is 4. Calculation of aperture using guide number and distance.
I can't even understand how one would use a flash meter in situations 1 and 2 where there is either kind of internal flash metering system being used.