What are you trying to install over this area? How much is the difference in heights?

Generally with self leveling concrete or floor leveling compound you will pour the whole area, because that's really the only way you end up with a level floor.

I just did a project trying to do just the low areas and ended up with a bunch of edges that even though I troweled them was not level.

So I advise pouring the whole area. Don't skimp to save a few dollars like I did....

SLC is actually for leveling, not to be confused with flattening. For most floors, you need them flat: no peaks or valleys across the length. But they don't necessarily need to be level: bubble on a level is in the center. A floor can slope from one side to the other as long as it is flat.

It sounds like you're trying to flatten the floor. For the low spots, I'd just add some wood floor patch to fill them in, then sand when dry.

SLC is used when you want the floor level, and it's applied over the whole floor. It also flattens it, by filling in the low spots, so you get a two-for-one.

Is wood floor patch basically the same as SLC? Or is it thicker? I have not begun the work but can feel lack of flatness in the floor. While this might be due to uneven mortar under the tile which I will sand down when prepping the floor, I expect pitches in the floor at various points.

Floor patch is different than SLC. Floor patch is something like this, which you spread on with a putty knife. It's kind-of like grainy wood putty and it can be sanded.
{{gwi:2133201}}

SLC is a cement-like substance that you mix in a bucket and pour onto the floor. It dries rock solid.

Here is a link that might be useful: DAP floor patch