Why do American subs have a less test depth in comparison to a German UBoat?

[A Soviet Fanboi]

Originally Posted by ETR3(SS)

As Bubblehead has stated (and I'll expand on) diesel boats most certainly can rest on the sea bottom. Most nuc boats cannot, or could not, rest on the bottom due to Main Seawater and Auxiliary Seawater suction and discharge. The exceptions I believe were SSN-575 Seawolf, SSGN-587 Halibut, and SSN-683 Parche. With no reactor plant to keep running, there's no need for a MSW/ASW system or to take a suction from or discharge to the sea while submerged in a diesel boat.

What's the difference between Main Sea water suction and discharge, and Natural Circulation of sea water in Nuclear submarines?