Which file and which part of the code controls the menu unlock?

[kimduck]

For example, if clearing the normal game unlocks the special game menu, or if a menu is locked on the first playthrough and becomes unlocked after meeting certain conditions,
I would appreciate it if you could tell me which file and which part handle this.

 

[Bloodbane]

The only menu unlock is by completing a level set and to unlock other level sets.

set    Secret
lives    8
credits 1
ifcomplete 2
...

This secret set will only be unlocked after completing other level sets at least twice.