If you fly IFR over the transition layer, you fly at a flight level.
Below FL245 you obey the quadrantal rule. If your magnetic track is 0 to 89 deg you fly at on odd flight level (eg FL50,70). If you fly a track of 90 to 179 deg you fly at odd flight levels plus 500ft (eg FL55,FL75). 180 to 269 deg is even FL (FL40,60). Final 270 to 359 is flown at (go on have a guess!)
Semicircular rule applies above FL 245. And I'm a cr@ppy PPL so I learnt it for Air Law and forgot it years ago! however it's not as straight forward as the quad rule I don't think.