Just to add to what the others have said, there are standard charts for the fits between holes & shafts ie. BS 4500 but, as mentioned before, this has to be tempered with a bit of experience. It is very easy to rely entirely on the standard charts but you might then over specify the components which inevitably results in increased cost.

It is perfectly reasonable to just ask the machine shop for a 'sliding fit' or whatever between 2 parts but you must accept that your idea of a sliding fit may not be that same as the machinists.

Of course, life gets a whole lot more complicated when you have say, a part with 2 pins which have to slide into another part with 2 holes & you get involved with positional tolerances on the features as well.