UK Subs have either a two-crew or an augmented-crew system. The two-crew subs are exactly what the name suggests, the single crew subs have about 150% of the crew they need take to sea, and of the 50ish extra people, some will go to sea as "additionals" to enable them to gain the qualifications and experience they need to fulfill their rôle, and some will be left behind to take leave or attend training courses. The larger part of these extra people are engineering types who tend to be busy carrying out maintenance when the subs are not at sea, and therefore do not have the same opportunity for leave/courses during these periods. For a "shooting war" none of the normal "additionals" are required as a necessity, but others (who may or may not be part of the crew) would be taken to strengthen the resilience of the crew. I would guess that it is this augmented-crew model which is being adopted by these ships.