i think topping from the bottom is one of those widely used loose bdsm 'terms'
what everyone has said would be considered topping from the bottom, but it depends on how you're particular r/ship works and what the individuals determine is acceptable or not.
as deigja said it can often be done without even the sub realising or intentionally doing it.

and how do we explain hard somethings we will never do limits? there the subs saying i WONT do this this this and that
or what about safewords? if a sub uses a safeword he/she is saying STOP isnt that also topping from the bottom in a sense? after all the subs the one with all the control in that situation.
if a sub needs a safeword in place with their Dom/me (putting aside play partys new partners etc) then you could argue that neither fully trust the judgement of the other in which case you're holding back somewhat and can never fully surrender total control/submission and if we take egs from previous posts unless you do, then that can also be considered a form of topping from the bottom.

but they're simply eg's and just my convoluted way of thinking lol ...its a vague term and imho they're really is no wrong or right or exact definition for it.