What aspect of a Roman soldier's duty cemented the link between the *salarium* and guaranteed periodic payment?

Answer

His duty roster was fixed regardless of daily output fluctuations.

The soldier’s *salarium* was fixed because his duty roster was fixed, meaning the payment was guaranteed irrespective of whether he spent that day drilling or on garrison duty.

What aspect of a Roman soldier's duty cemented the link between the *salarium* and guaranteed periodic payment?
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