I imagine it's things like the maximum power output of the power plant, the volume it runs at, things like that. Similarly, the max depth of submarines and their audible signature are closely guarded secrets.
E.g. if you exactly know reactor XY was assigned to Z submarine, and you see it in the boneyard, you could assume Z was decommissioned. And if you are e.g. China you could estimate how many submarines are deployed in your vicinity.