Complex sentence: As the P900 uses UIQ version 2.1, which is backwards compatible with UIQ 2.0 as found in the P800, applications made for P800 will usually work on a P900 as well.
Split into simpler sentences: As the P900 uses UIQ version 2.1 it is backwards compatible with UIQ 2.0 as found in the P800. Applications made for the P800 will normally work on a P900 as well.