Enlistment Contracts in the Roman Army

[Robert Vermaat]

+ some were conscripted forcibly. - that I take to mean "Shanghi ed"
+ there was a bounty for recruits - So there must have been bounty-hunters too.

If you think of folks being plucked off the street as in early 19th c. navies, that's not the case. Recruits were provided by landowners and towns. More recruits meant bounties, but there were no bounty hunters. recruiting officers might be a better picture. Of course, those who were 'provided' might object (leading to self-mutilations and laws against that).

+ does a soldier enlist for a single hitch (3 years, 5 years)?

No.

+ or a single campaign?

Not in the regular army, only if you are a non-Roman and you are hired for a campaign. The Goths were hired on that basis, and Alaric tried his utmost to change that into a regular job.

+ or Until he has completed the required years to qualify for retirement?

Yes. Even then he could be called up as a veteran under dire circumstances. His son had to enlist as a soldier too.

+ Does a Tribune bind himself for the same time span as a line soldier?

Yes and no. There were still officers who served for a short time, as well as men who rose through the ranks, even right to the top job.

+ Are there service committments that must be made once a soldier reaches a certain rank.
(Once made a Centurion he is comitted to remain until retirement)

Not that I know of.