PX is often used as an abbreviation for price in Bloomberg.
Fields prefixed with
PX are generally static fields: the value is requested only once and is based on whatever information is available when you send that request. On the other hand, real time fields keep sending new data as it becomes evailable.
As an example:
PX_LAST is the last price as of when you request it and won't change afterwards, whereas
LAST_PRICE is a real time last price which will keep updating if you subscribe to those changes.
Other examples include