Since an ISA is one kind of red sex link, the question, while important to the poster, is irrelevant, really. Please take offense at my saying that, as it is not intended to be offensive, merely to point out a fact.
There are many, many commercial strains of red sex link. ISA made a name for itself with its "Brown", but there are many others in the market place. Most of the top 6 or 8 poultry genetics corporations who sell eggs and breeding stock to the hatcheries, both smaller retail hatcheries or the larger commercial hatcheries for the egg industry, all have a version or two of these top laying hybrids, they are quite common in in backyard flocks everywhere.
Fred is one of the most knowledgeable chickenkeepers I've ever run into -- you can trust what he says is right.
But it can still be confusing! I know, I've been there (maybe still there)!
Basically a red sex-link is a hybrid -- a combination of chicken breeds (there are a number that will work) whose genetics result in sexable chicks at hatch. They were developed for the egg-laying industry. You can count on them to produce lots of extra-large/jumbo eggs.
The ISA Brown is one specific mixture created and owned/patented (don't know the legal lingo) by one company: ISA. Only their birds can be sold under the ISA Brown name. Lots of red sex-links will look exactly like them, and may in fact be as near as makes no difference to the patented ISA Brown. Picture red sex-links as cola, and ISA Browns as Coca Cola. A specific name brand. Other companies have Dr. pepper, Faygo, Pepsi, etc.
If you really want to know if you have Coca Cola in your chicken bottle (so to speak), ask your store which hatchery they buy from. Then look up that hatchery online and see what they sell. Last I heard, only one hatchery in the U.S. sells ISA Browns -- Townline, in Michigan. Quite honestly it probably doesn't make a lick of difference.
Fred, please let me know if I've got any of this wrong. Thanks!