There are so many interesting tidbits tucked away in the code, things the devs had to consider that have never crossed my mind whilst playing. For instance, the idclev cheat handling in st_stuff.c has to check if the game is shareware and restrict level changes beyond a certain range.
I never quite grasped how that shareware check worked. Always thought the WAD file was what you bought, not the engine/executable itself?
From doomstat.h line 60:
// Game Mode - identify IWAD as shareware, retail etc.
extern GameMode_t gamemode;
I haven't done a deep dive on the code, but it looks like gamemode is set based on something in the WAD file.