"rxgain" versus "VM_GAIN"

What is the difference between:

the “rxgain” value in "/etc/asterisk/zapata.conf"
the “VM_GAIN” value in “/etc/asterisk/extensions_additional.conf”

I want to slowly increase the volume of voicemail
messages and stop when there is distortion.

rxgain/txgain adjust the overall gain for the channel, making it louder or softer for all things.

vm_gain adjusts recording for voicemail only.

When did vm_gain become a part of asterisk? I have read about the patch, but didn’t realize it was in a release version of asterisk.

im assuming its a trixbox thing… a vm_gain variable that is inserted into all the Voicemail() priorities.