2. ABR. This is new in Leopard, and is what you now get in iTunes when you uncheck the VBR checkbox.
3. VBR_Constrained, a form of variable bit rate where the variation in bit rate is limited. This is what you now get, and apparently have always gotten, in iTunes when you check the VBR checkbox.
4. VBR. This unconstrained, low-complexity form of VBR is actually new in Leopard
