I don't quite get the usage of the stack here. Is it simply an organizational method, where each draw will go through whatever is on the stack and use those transformations, or is there something along the lines of an ongoing product matrix, which keeps getting multiplied by transform matrices and then their inverses when popping?

I don't quite get the usage of the stack here. Is it simply an organizational method, where each draw will go through whatever is on the stack and use those transformations, or is there something along the lines of an ongoing product matrix, which keeps getting multiplied by transform matrices and then their inverses when popping?