The percentiles can be found in this link appendix b
http://www.nwea.org/sites/www.nwea.org/files/resources/NWEA_2011_RIT_Scale_Norms.pdf

You want the scores to be going up and the percentiles to remain constant. When scores stay stagnant or go down it may be because the child isn't learning anything new. It could also be the child had a bad test day.

MAP tests are used by many districts to determine placement in gifted programs and accelerated classes or grade enrichment.