#40 tapers?

Studying the specs on the Bridgeport series 2 (What else are you suppose to
do while waiting for it?) it list the spindle as No. 40 milling machine.
When I look in the supply catalogs I see Cat 40, BT 40, ISO40, and NMTB 40.
What are the differences and are any of them interchangeable. Which one
really fits the Bridgeport?

Tim
[Denver, CO]