Raiden V: Director's Cut (EP)

All routes are dependent on the hit ratio:

S route = 98%
A route = 95%-98%
B route = 90%-95%
C route = 80%-90%

Stage 1 only has 1 route, Stage 2 only has A and B but Stages 3-8 all have S-C routes.

Keep in mind that if you start route A, you can't go to route C. For instance when you wanna unlock the stage 6 boss ending. You must start on stage 4 and get below 90% hit ratio on both stage 4 and stage 5. You can also see what route you are on, on top of the screen to the right. If you followed Step 1 of the Road Map then you should have unlocked all the S routes so just work your way on rest of the routes through Chapter select.