So basically those are values calculated by MusicBee which are applied during playback if "track gain" is ON? Am I getting this right?
yep
Nothing to do with Reply Gain, correct?
I don't understand what you believe that 'Replay Gain' is exactly.
The literal 'ReplayGain' is ancient and a bit obsolete, but the concept is the same as what MusicBee is using today, be it in a much more modern and better way. (using the EBU128 algorithm)
It may be good to do some serious googling and reading on these search terminologies.
There is a vast amount of information to be found on the matter.
One thing that I believe confuses you is the difference between the audio data of a track being permanently altered to a different loudness, vs. only adding tags to it so that the player can handle that at playback.
Both ReplayGain, and MusicBee can do both.