You can set up any Units of Measures (Square meters, Acres, Hectars, Kilos, Units, Pieces etc...) You define them.
Not sure why You want to use Job's module just to do annual billing, as Recurring Sales Lines / Invoices would be a lot easier. But if You use JOB module for other functionality, then You can add a planning line to create an invoice - You just have to add it for each year.
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