The arms, equipment and impact of Late Roman Clibanarii

[Renatus]

While those cataphractarii were actually no good at all?

Of course not. Just because one unit may be particulary good at what it does, does not mean that another unit is no good at what it does.

You see, I do not mean similarly armed, it could be a whatdoyoucallit, two words for the same thing.

'Tautology' may be the expression that you have in mind. I am afraid that I cannot agree with you. Cataphractarii and sagittarii are completely different types of unit and are attested as such. In fact, unless you were to argue that they were similarly armed and organised (which you say that you do not, at least so far as armament is concerned) and, therefore, one and the same thing, I cannot see how using the two terms can possibly be tautologous.