I'm no expert either (we might need Mac to weigh in here), so I may not be using all the correct terms below. But I never let my ignorance keep me from talking anyway.

I think the only legally restricted tartans are the copyrighted ones, or whatever you call it when you commission a custom designed tartan. Some examples are corporate tartans and private individuals who register the tartan and do not release it for public use.

For example, when my acquaintance, Robert Hall, designed the Leatherneck tartan, he did not reserve or restrict its use, and so anyone can weave or wear it without paying a royalty

I think the above pretty well outlines the actual restrictions on tartan. There are many traditions and such which are not legally binding but are often observed.

But what do I know?