"Acid" flux is the stronger class of flux; it has something like hydrochloric acid in it. (The paste form has zinc chloride.)
This
is good for making difficult oxides dissolve so difficult metals like stainless steel can be solder-wetted.
But the acid can hang around later trying to corrode the metal it just cleaned for you.So for electronic stuff we mostly do not use it.If we do, we scrub it off with things like toothbrush, water, soap, alcohol, baking soda, to minimize acid residues.
The flux built into most solder wire is called rosin flux.
I think it is an organic acid (so is vinegar, and tart-tasting "citric acid"),stuck onto larger molecules that melt only at
soldering temperatures.That is the clear yellow-brownish plaque that sits on the solder''s surface when you are
done.It does the same stuff as acid flux, but it is milder two ways.It is only strong enough to reduce weakly oxidizable metals like copper, tin, lead, silver.So it is just strong enough for electronics use, but not for soldering to stainless
steel or iron or anything with chrome or aluminum.
And rosin-flux goes back to its plastic-like solid form after use, so it does not act very corrosive to the metals later on.
So we do not need to clean it away carefully.It can be cleaned away if you want to work at it, with brush-scrubbing and the right "polar solvents"."Flux remover" is sold in spray-cans for this. Rubbing alcohol with a dash of dish-soap
sort of works for me.
Acid flux
residue may not loose its component for quite sometime and may be hazardous.