The following was from the USTA web site and I recently amended it to reflect more recent changes:

 

 
As of January 1, 2006, the Points Per Round Ranking System shall be amended to reduce the number of events to be counted for women’s rankings from six (6) to four (4) events. The number of sectional (250 point) championships which will be counted for national ranking purposes will be reduced to two (2) events for men and women. All sectional championships in excess of two (2) will not be counted as sectional championships for national rankings,but can be counted as local tournaments for national ranking purposes.

All sectional championships in excess of two (2) will  be awarded national ranking points as local tournaments (125 pts. to winner).  This does not impact local rankings, this limitation is applicable to national rankings only.