ur welcome.
assuming that when all the pads, shoes, disc, drums were previous new/thick that the reservoir began at Max and there's been no fluid leak/loss - as brake material thickness gets worn thinner, more fluid is required to push the pistons further in over time therefore the reservoir level gets lower.
once the level reaches Low and triggers the low fluid sensor/light it's a useful hint at telling you that ur pads/shoes might be getting thin.
generally if pad material is still within spec and there's no leaks and the fluid level is above Min, I'd usually just leave it alone and not top up, allowing it to work as I mentioned the paragraph above.