-AR and -PI are suffixed to part numbers which indicate the display module in question is in a Kit for Arduino (-AR) or the Raspberry Pi (-PI).

In the kit will be the display module in question, plus (but not limited to) the Arduino Adaptor Shield (for -AR) or the Serial Pi Adaptor (for -PI).

For example, uLCD-43DT-AR is a uLCD-43DT display module in Kit form to includes parts to interface easily with an Arduino.

The display module used is identical whether you buy the display module by itself, or you buy it in the -AR or -PI kit forms, the kits just include some extra parts for easy interface, while the display module by itself does not.