Tesserarius - rank or role?

[Sardaukar]

But I don't think the tesserarii would have been supernumerary to the rest of the century, if that's what you mean, any more than the musicians, for example, the legionary cavalry or those detached for admin duties at headquarters.

I doubt too, that a roman century had supernumerarii, except the centurio himself. There is no evidence for more than 10 contubernia or a special room for the principales in roman infantry camps. Well, there are some hints in cavalry camps.

However, due to the fact, that a roman legion was never fully staffed, it could have been easy to group the principales (signifer, optio, tesserarius and perhaps custos armorum) in one of these 10 rooms. I doubt, roman privates liked the idea, to live in the same room with a seargant for years more than modern privates do.

I am even not sure, if e.g. a cornicularius consularis, was a supernumerarius. Fact is, he was still on the payroll of a century. O course, no legate with a brain, let his cornicularius fight with his century. So perhaps only the additional centurions (there were up to 11 centurions in one cohort) were real supernumerarii.

Maybe optio could have been supernumerarious too, but I agree that signifer, tesserarius etc. were most likely not.