My definition: Pro rata, when it comes to jobs, generally refers to pay. Say the salary is '$40,000 per annum-pro rata', it will mean that if you only work part time, you get part of the pay.A standard week is (about) 40 hours, so the pay for 20 hours will be $20,000 per annum.y definition: Pro rata, when it comes to jobs, generally refers to pay. Say the salary is '$40,000 per annum-pro rata', it will mean that if you only work part time, you get part of the pay.