Some valves have an allowable leakage rate. These rates differ depending on what organization sets them. Common organizations for allowable leakage are API, MSS, and ASME. Depending on the diameter of the pipeline, the leakage rate is determined. If the valve leaks even a drop more than the specified amount, it fails the pressure test and therefore does not meet the standard in question.
Some valves are made with the purpose of eliminating leakage of any sort. More often than not, soft seated valves are able to achieve zero leakage easier than metal seated valves because of the pliable seat. It is much more difficult to achieve zero leakage with metal seated valves, as the seat materials must be perfectly form fitted.
Bubble tight valves do not pass bubbles when they are fully closed. Bubble tight is when one part of the valve is in water and the other is pressurized by air/nitrogen. The test standards look for the bubbles in a specified amount of time. If the bubbles are released, the valve is not bubble tight and has failed the test.