>>96386946
In the fluff, veteran sergeants are the lower class of line centurions. And they can lead just a squad or a whole small detachment. Like the chap in pic related.

Actual basic sergeants do not have crests, just a stripe in the helmet or that RT-style chevron icon somewhere in their armour. But when you put a squad of miniatures in a table, a crest makes it easier to read which mini is the leader