Really does depend on the company - LX used to use the old SR logic of country group / airport / sequence in day - Air France used to use the thousand blocks of 3xxx, 5xxx & 7xxx to tell you whether the fare was Red, white or blue and the xxx as sequence within day at airport from a given airport in Paris- so an airport might have had the incomming flights as 7351, 5353, 3355, 5357, 3359, 5361, 7363 for example for the 7 daily flights inward from Paris ORY, add 1 for the returns and adjust 1st figure if required for the fare basis.
Guess the best rules are your own rules.
