theres a certain degree of trial and error too. Lets say u has an epic disc you wanna use for a boss battle like Falz on hard+ and u use the disc at the spinning spikes stage...youre all hyped up and happy til the Final form, suddenly its default battle BGM. Same wiff Olga and ep4 boss stages. You'll also notice each area on pioneer has its own BGM. Shops, hospital, office, guild.... use a disc out at the bank area/etc then walk into shop....poof! music go bye bye =/.