The words "workstation" have no set meaning anywhere in the music world. But Diki is right in that there is a general understanding about the limitation of arranger "workstations" and pro "workstations. But in all honesty the difference relates more to how you use the keyboard than what the keyboard is.

The true difference between an arranger workstation and a pro workstation is the degree to which you can manipulate the music, notes ,sounds and effects to come up with a completed piece of music. In reality both types of instrument rightly deserve the title "workstation".