Flow / RU (residual urine) is an abbreviation for the combination of the urinary flow test with the post void residual volume test.
Urinary flow test. You urinate into a receptacle attached to a machine that measures the strength and amount of your urine flow. Test results help determine over time if your condition is getting better or worse. It is important to have a full bladder and a strong urge to go for this test. On a scale of 1-10 where 1 is no urge and 10 is an accident will happen any moment, it is preferable to have an urgency to urinate of 8, 9 or 10.
Postvoid residual volume test. This test measures whether you can empty your bladder completely. The test can be done using ultrasound or by inserting a catheter into your bladder after you urinate to measure how much urine is left in your bladder.