TC aux. Who where they?

[Ti Cl Nero]

What kind of unit are "Symmachiarii" ?

Literally, if I am not wrong, symmachiarii means "those who fight together", "allies".
Anyway, it is not yet clear the meaning of this title in the context of Imperial Roman Army , and many scholars debated the topic.
Symmachiarii units are mentioned in the De Munitionibus Castrorum(19,29,30,43).
This source has been also compared with an inscription (AE 1926, 88 ) where a unit of Symmachiariii Astures raised for one Dacian war (not stated which one) is mentioned.
Most of the uncertainty about the Symmachiarii's role and chronological context in the Imperial Roman Army (particularly in trying to relate them to other attested types of ethnical units like numeri or nationes) comes from the difficulty in dating both De Munitionibus Castrorum and this inscription, it seems.
Regardless of the dating problem, anyway, P.Southern ("The Numeri of the Roman Imperial Army", 1989) , following E. Birley (in "Antiquitas", 1976), suggests that Symmachiarii may have been temporary ethincal units levied for the duration of a campaign (as opposed to numeri, which were permanent ethnical units).
Just a possibility, but it seem a good one to me.
Vale