Estimates based on your current rank (default 7.0), your estimated standing (default 50) and an estimate for the total number of players (default 500).
Some private servers reduce the amount of RP required for a certain rank. You can control this with the rpfactor parameter (default 1.0, neofeenix would be 0.67, fenix would be 0.5).
All parameters are passed as URL parameters, like this:
?rank=9.40&standing=50&players=600&rpfactor=0.5
A rank of 9.4 means that you are rank 9 and 40% progressed towards rank 10.
The RP/rank gain estimation is more accurate the closer you are to the center of your bracket. The closer you are to the top of the bracket, the higher the estimation could be (meaning you will probably gain less). The closer you are to the bottom of the bracket, the lower the estimation could be (meaning you will probably gain more). It also depends on how close or far the players at the bracket borders are CP-wise: The greater the gap, the more gain for the bottom player(s) in the higher bracket and the less gain for the top player(s) in the lower bracket. The smaller the gap, the more less for the bottom player(s) in the higher bracket and the more gain for the top player(s) in the lower bracket.