I think it is best to compare similar things. So first let's start with grit or particle size. So for instance let's look at a strop with 2 micron CBN (Cubic Boron Nitride). That's 8k grit. Or a 30k stone - that's a half micron particle. Obviously the 30k stone will give a finer finish than the 2 micron strop. So if you are using a 16 micron (1k) CBN strop, you will have an aggressive cutting strop. And so on.
So you can begin using a strop after a 500 grit stone using coarse grits on a strop - or go up to a very fine stone like a 30k before using a very fine grit strop - quarter micron tenth micron etc.
A first consideration is just how refined of an edge you need. This is largely a matter of personal taste and should be decided depending on a number of factors, the most important being the intended task for the knife.
My personal preference is to just simply think of stropping (a vague term) as what you usually switch to after your finest stone and just to think of honing or sharpening and stropping as simply abrasion. So I prefer to not make hugh jumps in abrasion from a coarser grit to a finer grit in any case. So for instance if you are using a 10k stone (about 1.75 microns) going to a 24 k strop (0.75 microns) is a nice jump. Still aggressive enough to continue the process rather than say jumping to a 0.25 micron or 64k 'strop'.
So the next time someone says they have a 6k edge but they stropped it on something finer - well it isn't a 6k edge anymore.
Other factors are the type of compound and the type of strop. So for instance a 0.75 micron CBN strop will cut surprisingly aggressively - especially so on abrasion resistant steels. I use this for touchups on a reasonably sharp edge on kitchen knives for my own use. It's a dry technique and quick. Quicker than splash and go - just Go
I'm going to avoid going into detail here just to leave the emphasis on particle or grit size as the most important consideration rather than if it is performed on a strop or a stone.