Sassanid Infantry

[Bashir]

and don't forget the Romans too draft their subject into the army too.

1. The Roman subject peoples were separate from the professional army, as Auxilia and later Foederati. While the Foederati were an allied militia, the Auxilia were professional soldiers.

2. The Romans had a truly professional army, just like a modern army. The soldier had a full-time job of being a soldier, was paid by the state, and supplied his equipment by the state. The Sassanids did do this as well, but not nearly to the degree the Romans did.

3. Conscription is a method of recruitment, it means being ordered to join the army. It does not mean what kind of army it is. Militia are conscripts just as much as the professionals we drafted to fight in WWII and Vietnam were. The Romans still retained a mostly volunteer army well into the 4th century AD.

4. Nadeem Ahmad, who posts here on occasion, could explain to you how the Sassanid army worked in quite extensive detail.

The legionaries are professionals, then what position auxiliaries held?

Militia was different from a conscript:

Conscript

Enlist (someone) compulsorily, typically into the armed services

Militia

A group of people who are not part of the armed forces of a country but are trained like soldiers.