See above. That's basically the case. There are a few nuances to consider.

If some authority has declared the tartan to be official, then it is not a fashion tartan. For example, if the cheif of the Robertson clan puts his stamp of approval on a particular tartan, it becomes a Robertson clan tartan. If a city council decides to adopt a particular tartan, it becomes that city's district tartan. And so on...

Strictly speaking, a fashion tartan is a design that someone has given a name to that has no real authority behind it. Sometimes, however, a fashion tartan becomes a de facto clan/district/etc tartan through wont and usage.

One example would be the Aberdeen district tartan. It's never been officially adopted as the district tartan for the City of Aberdeen. But it's been around since at least 1794 (that we know of, possibly earlier). And since that time it has been worn to represent the Aberdeenshire region. There is a long history of use with this tartan.

Another example might be the Irish County tartans designed by the House of Edgar in the mid-1990s. They don't have the history of the Aberdeen tartan, but they are very popular nonetheless. Even though they have never been officially adopted by the Irish government, people are wearing them to represent those counties, so in effect they are being treated as district tartans.

I don't beleive that a case could be made for the Rannoch tartans being anything other that fashion tartans, however. They have neither history nor usage behind them. On the surface, it would seem that they are simply corporate designs that the company decided to give a pretty name to. I could be wrong in that assumption, but I didn't see anything in the record to indicate a broader usage than that.