They should relate to the streaming bit rate but they don't work properly.
1516 = SD
7830 = 1080p (Boost pass needed to achieve)
128 = 720p (This is obviously not the bit rate but it is what is shown - this should be seen most times if you don't have a boost pass)
You can get a few different ones in between as the quality stabilises but the three above are usually the main ones when settled.
When I'm watching something on catch-up on the Now TV App on my DTR2100, if I hit the info button or pause the action you get all the usual on screen stuff, but up in the top right corner it's also showing a stream quality figure.