Hi, Barry. I'm glad to answer this one. Real thinset mortar is a cement-based product: you have to add water to a dry powder. Only some of the water evaporates as the product hardens: most of the water is broken up into hydrogen and oxygen molecules and, during the course of the reaction, bonds with the molecules of different compounds in the powder to form new molecules. In other words, real mortar hardens because of a chemical reaction.
I'd say the consistency of mortar can best be described as smooth peanut butter...with some sand added in. I know that's a weird thought, but please bear with me.
Mastic is a sticky tile glue developed as a convenience product. It is approximately the consistency of regular creamy peanut butter (no sand). It hardens as it dries, like most glues.
This product, marketed as "pre-mixed thin-set mortar" is a tile mastic that mimics the consistency of mortar. On other words, it's like mastic with some sand added in, and that's how many experienced installers would describe it. Many of us look down on this product as a trick or cheat, designed to take money out of the pocket of the unsuspecting DIYer who has been told s/he needs thinset mortar, but wants a product that can be used more conveniently than the powder in a bag. You will notice that the term "mastic" never appears anywhere on the packaging of this item or in its online literature. But that's what it is, tile mastic with some gritty particles added in to mimic the consistency of real mortar.
All the same, there are legitimate professional uses for this product. The sand in the mastic makes a slightly thicker base of it easier to install. This is important in situations where walls haven't been leveled properly before tiling. This stuff, judiciously applied, can save the day in those situations. I'm sure the manufacturer can think of others. I believe I read somewhere that the gritty particles in this aren't actually sand, but a crystalline drying agent that allows the mastic in the rest of the product to dry more thoroughly than in non-gritty mastic.
But the fact remains that tile mastic was originally designed as a convenience product. This stuff is tile mastic, and I feel its use should be clearly limited to decorative wall tiling in areas that are not exposed to liquid flowing over them. It should not be expected to bear significant weight, or withstand extreme heat. I would not use this for any floor installations, countertop installations, tub surround installations, shower walls, any exterior applications, or fireplace installations. This is just my professional opinion, of course. Others use it for a number of these things and have rarely had problems with it. But in all those cases, I use real thinset mortar that you have to mix out of a bag, and not this.
The last thing I can barely believe is that this stuff is now sold in gray and white color varieties, just like real mortar. The reason there are two colors of mortar is that there are two colors of cement, and the white stuff is more expensive to produce than the gray stuff. I can't believe that Custom Building Products, a very reputable tilesetting company owned by Quikrete, would go to such lengths to trick their inexperienced DIY (most vulnerable) customers into believing this is any sort of real mortar and paying a heavy premium for it, but they apparently have.