I found this explanation: Submarine escape and rescue: a brief history
I did my initial escape training with the RN at SETT in Gosport in the 70's and later with the USN at Groton. At the time, the major differences were that the RN did 'free ascent' training at 9 and 18 Meters as well as pressurized escape at 18 Meters. The USN only used the Steinke Hood in training (up to 100 FSW).