There is not a problem, Don, since signature IS unique to those killers who must use one. I say "must," because they have to employ signature to satisfy some need within themselves.
The easiest way for people to understand signature is to look at it as the trademark of a particular killer.
Too many people here seem to think finding pubic hairs at separate crime scenes is 'signature,' when in fact, pubic hairs, like white cotton fibers, are too common to indictate signature.
Uniquenss is one of the aspects that make signature the signature, just as our individual fingerprints are our own unique 'signature.'
There are also people on the internet who say JonBenet's murder is an isolated event when we do not know that as fact. Yes, she was removed from her bedroom as were so many other young children. Yes, she was found bound with ligatures just as other children have been found and yes, she had tape on her mouth--but people seem to expect an exact replica, including the note, to point to her murder being part of a series.
Sometimes the only thing that links a series is the manner of death or the events prior to the death, such as torture. Not particularly torture by knife, garrote or water torture, but simply torture. In that regard, torture would be the 'signtature.' Some sadists choose to use pliers one time, a knife the next, and a hammer next time around. It's all still torture-- the trademark, or hallmark, if you prefer.
The specific label isn't important if the idea of 'trademark' clarifies what constitutes signature.