A volume in Midi is a number in the range 0 to 127.
For example a Stop in Box Of Stops playing at three quarters volume would have a level of 96.
The other factor in how loud a stop plays is the Master volume - the volume applied to the whole Registration.
This means the actual volume a stop plays at is combination of both these volumes. For example if both the stop and master volumes are at half volume (64), they combine to play the stop at quarter volume (half of half volume), so the level is 32.