craigb
It's the amount of time you can reasonably expect the drummer to be on time (with 1/3 of the time being behind the beat and the remaining 1/3 being ahead of it).
HTH.
Actually it's funny you should say that but the only reason I'm a bit familiar with the notation style at all is because it gets used quite a bit where people like me are notationally challenged.
In a situation where nobody has the time to write down fancy charts and chord symbols it's really easy to quickly write down some changes that everyone quickly gets, (including the drummer) so you'll get some amendments to a score someone will give you a piece of paper with the changes on then it's easier to say come in for 12 bars on the 2nd 5 for example.
Of course those who know what they are doing, unlike myself, can transpose easily too without needing to re-write anything.
It can be a real time-saver when time is money or you've only met some players half an hour before you are due to play and they want to do a few things differently.
It can make it seem like you actually know what's going on...