I was a little confused when I initially read this so wanted to add some clarifying details to the current explanation.

(My understanding from reading the current explanation is that it makes it appear as though apoptosis only occurs in negative selection and that is what the difference between the two forms of selection, which is not the case, so just wanted to clarify in case it helps other students)

Reference: https://www.khanacademy.org/test-prep/mcat/organ-systems/the-immune-system/a/adaptive-immunity

"Positive selection ensures MHC restriction by testing the ability of MHCI and MHCII to distinguish between self and nonself proteins. In order to pass the positive selection process, cells must be capable of binding only self-MHC molecules. If these cells bind nonself molecules instead of self-MHC molecules, they fail the positive selection process and are eliminated by apoptosis.
Negative selection tests for self tolerance. Negative selection tests the binding capabilities of CD4 and CD8 specifically. The ideal example of self tolerance is when a T cell will only bind to self-MHC molecules presenting a foreign antigen. If a T cell binds, via CD4 or CD8, a self-MHC molecule that isn’t presenting an antigen, or a self-MHC molecule that presenting a self-antigen, it will fail negative selection and be eliminated by apoptosis."