The term "universal" originally referred to a universal tartan worn by Scottish soldiers in the British Army, the aforementioned Independent Highland Companies that later became the Royal Highland Regiment (the old 42nd & 73rd regiments of foot), the "Black Watch". It is only recently that the "universal" or "government sett", as it was known, truly became an "open" tartan, mostly due to marketing.

The same with the Stewart Royal sett, which was originally a tartan which was worn only by permission -- Pipers in the Black Watch & King's Own Scottish Borderers, for instance, wear the Stewart Royal.

The Stewart Hunting has little connection with the Stewarts, but is another military tartan which has become "universal"; the Royal Scots, the oldest regiment of the British line (raised under warrant from Charles I in 1633 for service in France) wears the Stewart Hunting (as trews).

There are also tartans like the Caledonia & Jacobite tartans, for anyone one to wear, and the aforementioned Flower of Scotland, Scottish National, etc.

District Tartans for Scottish districts, Canadian provinces, US states, etc. are a great alternative to clan/family tartans, and do not receive the attention they deserve, since they are a much more older tradition than the clan tartan.

Bottom line: unless it's Balmoral, or a tartan that can only be purchased from a Chief or Clan/family society, then it's "open", BUT, always "know your tartan" and the history behind it.

Cheers,

Todd