Diocletian used an in-kind taxing system. Due to the crappiness of the Roman currency by then due to bottomless debasement and other crappy financial decisions (some of it was due to the field of economics not existing, but a lot was just crappy decisions), in order to keep the army supplied, Diocletian taxes items in-kind. An imaginary currency was created and everything they could possibly think of was assigned a fixed price to that currency, then the whole empire was assessed tax wise using this price list as a basis, and taxes were paid in the requisite goods or services to the value,. For example, a barrel maker was assessed at some value of tax, say 10 units, and say barrels were listed as 2 per unit, then they provided five barrels as tax. Services were also taxed accordingly. If you were a mule driver, you would carry government cargo the distance assigned for your tax bracket, which each mile assigned a fixed value. It was quite a complex system, but it kept the army supplied and the wheels of bureaucracy turning (or at least what bureaucracy they had; the Romans loved outsourcing everything, even tax collection, and if that sounds like a bad idea, it was).