It is designed to be varnished over. It can be used as temporary protection if you have to show a painting before it is fully cured. Then when you are ready for a final varnish, carefully give the painting a quick wipe with a solvent to remove any built-up dust and do a final varnish. Some artists also use it during their painting process to oil-out sunk-in areas, but many brands specifically mention that this isn't its actual purpose. But it does add gloss to matt areas.
Using Retouching varnish
Updated over a week ago