D2X-XL uses a sophisticated standard algorithm to determine the effective "room size" at any given segment of a level. Based on that, you can determine at what room size reverberation effects should start and at which room size it should work at full strength. For rooms between these sizes, the various effects (initial reflection, amount of echoes, muffling, time to decay etc. will be scaled). For rooms above the largest room size set by you, it will do maximum effects.
The values the percentages relate to have been determined by looking at the various distances present in D2X-XL and what distances suggest which effects. I have tuned the default settings to my taste; If you find they don't work well for you, just play around with the settings unless they fit.
You can of course also turn reverberation completely off if you don't like it.