A file with only Reg F service is very straightforward to review & audit. Similarly, a file with exclusively Res F service is straightforward, but not quite as simple as a Reg F file.
The files with a mix of both are more complex, and care is needed to ensure no time gets counted twice For example, a Reservist with 12 years in joins the Regs, and received RFRG for those 12 years, and now claims PiL; those 12 years must be deducted from the calculation, but any fractional time left over in their Res service would be counted.