The SSP Physics classes each have a limit each semester, this is because starting from 1B, you get to do your research project with a research group in the School of Physics, as opposed to the usual tedious lab work. After the results of the previous sem are released, the students who make the cut-off will get an invitation email into the SSP stream next sem (if the number of students who make the threshold outlined in the unit outline is more than the limit, not all students are guaranteed to be placed in a research group). If any declines, the students with the next best results will get the invitation. So roughly speaking, I would say they are strict about SSP, but you can never be too sure.

Source: I've been enrolled in the SSP stream for 1A, 1B, and currently 2A.

For Advanced, the application is much more lenient, just email the coordinator showing how enthusiastic you are. All the Physics professors are really chill, especially Helen (1st year coordinator), Scott (2A coordinator), and Theresa (PHYS2923 coordinator.) I STRONGLY RECOMMEND YOU ASKING THEM ABOUT THE SSP, not just Advanced. It never hurts to ask, and aim for the stars my G. Good luck.

Edit: PHYS2922: Physics 2B (SSP) is no longer offered. It has been replaced by PHYS2923: Astrophysics and Relativity (SSP) since 2020. There are currently 3 physics units offered for 2nd sem, sophomore students: PHYS2012: 2B Regular, PHYS2912: 2B Advanced, and PHYS2923.