This really serves no use anymore and is mostly a remnant of old times with dark CRT monitors. The default ambient level was set at 20, meaning a sector light level of 40. This is a value actual levels rarely get to, except when using some lighting effects - but it's for those that the ambient clamping did the most damage.
- renamed menudef.z zo menudef.zz.
This was done because some tool out there stupidly assume that *.z is some sort of archive and refuse to operate on these files (shame on you, Beyond Compare!)