Confusion regarding SDL_LockTexture

Hello, I think I am misunderstanding SDL_LockTexture’s rect parameter. When passing a rect which is a sub-size of the entire texture, is pixels still meant to contain all the pixels of the texture? When I pass a rect I see no difference from passing NULL. The wiki says pixels should be offset by the locked area (the rect I’m assuming), but the results seem to conflict with the description. Am I just misunderstanding how this parameter is meant to work?