There's a reason for "both" .....
It's not as quite as simple as a one word answer, although both is correct, here's why ....
The miter slot is the only part on the saw that is not moveable or adjustable, unlike the blade/arbor or the fence.
So, you must reference all the adjustments of the blade/arbor and the fence to the miter slot which is milled into the table. The blade is mounted on an arbor which in turn is either table mounter or cabinet mounted depending on the type of saw. There is a different process for adjusting the blade/arbor on a contractor saw which is table mounted than on a cabinet saw which is ... cabinet mounted.
The fence is the easiest to align with the slot.
The arbor /blade is more complicated, because of the access to the bolts, and because tightening the bolts sometimes throws off the adjustment you just so precisely made... :frown2: and also because you are measuring from the front and rear of the blade over to the miter slot for each slight movement of the arbor. It can be tedious...
By aligning both the blade and the fence parallel to the miter slot you end up with the blade and the fence being parallel to each other, by "default".... so "both".
The answer to your question will only be as detailed and specific as the question is detailed and specific. Good questions also include a sketch or a photo that illustrates your issue. (:< D)
Last edited by woodnthings; 06-11-2017 at 01:31 PM.
Reason: bottom paragraph added