It's never, getting any better than this.

For those who don’t know, the various patterns of tartan actually hold special significance to Europeans. Different styles of tartan are used to indicate a person’s allegiance to a certain clan, family, or order. It is even illegal in some places to wear specific brands of tartan without permission from the person in charge of the group to which the pattern belongs.

This being said, Aziraphale wears a very specific style of tartan. It’s called “Heaven’s Dress Tartan,” and it was designed specifically for Michael Sheen to wear in the show.

The naming of this tartan, along with the fact that it is the only tartan we see him wear, would indicate that this pattern is actually the official tartan of heaven. But as we see when Aziraphale is handed his military uniform and told to go assume command of his platoon, (they wouldn’t be waiting for him unless he was their Sargent - fight me,) Heaven has its own official Tartan, and it’s not the one Aziraphale is wearing.

Furthermore, when Aziraphale is discorporated and winds up in Heaven, the Heaven’s Dress Tartan on his bow-tie is gone.

This leads me to conclude that, since Aziraphale is wearing a unique tartan that does not match heaven’s, our dear Principality made his own tartan. He didn’t see that pattern somewhere and think it was nice, and he doesn’t wear it to show his allegiance to Heaven. This is A.Z. Fell’s clan tartan. He designed it for himself to wear, as his symbol. If he had a family, it would be their family tartan. 

Now, a thing about clan tartans is you’re only really supposed to wear it if you’re a member of the clan; i.e., a member of the family. Which brings up an interesting point:

Twice in the show, Aziraphale gives Crowley the Heaven’s Dress Tartan. The first is when he gives him the holy water. (Isn’t it sweet, the vessel he uses is decorated to represent himself?) This could perhaps be dismissed under the theory that it was simply what Aziraphale had on hand - it could easily have been any other securely-shut cup. 

However, in the scene where Crowley and Aziraphale meet Anathema, Aziraphale miracles a tartan bike rack onto the Bentley (which I couldn’t find a picture of). Aziraphale is willingly giving Crowley his specific tartan. Now remember what I said about clan tartans only being worn by members of the clan?

There’s different types: unnamed/standard, ancient, weathered, hunting and dress,

My clan (McCullough) has standard and dress and I’m permitted to wear McDougall tartan as well as we are a sept.

Basically what I’m going with here is that the tartan Aziraphale wears is the dress. It’s formal and designed to stand out a bit. My clan’s dress tartan has white in it, rendering it useless as a hunting (camouflage or war) tartan, while our standard has quite a bit of green. Tartans that are predominantly useless as a camouflage pattern (my brother’s is Gibson and he has a hunting variant rather than a dress variant) have a greened up version as a hunting variant.

Heaven, in show, is very…white. So the hunting variant would be paler to blend in. Dress would be darker with eye catching additions (I.e. red) to indicate that it is a) formal and b) non-aggressive.

Aziraphale has gone and made himself a formal non-aggressive variant of heaven’s tartan and wears it very obviously and shares it. Look at Warlock’s bed.

Here’s another thing - if your clan has a chief then you need their permission to gift the clan tartan to non-birth family members. Armigerous clans (like mine) need only an individual’s permission to share their tartan.

Basically, I can let my brother use my tartan but he cannot let me use his without the permission of his chief.

Wally: I dunno about you guys, but my parents don’t play.

M'gann: My parents abandoned me, and one of them is wanted for intergalactic war crimes.

Dick: Dead.

Conner: Complicated.

Artemis: One of mine tried to kill me.

pretends not to give a fuck but are actually cringing deep down inside about that awkward thing they did years ago: taurus, virgo

has no chill whatsoever and does not give a fuck: gemini, leo, libra, aquarius, aries

nicely gives a fuck out of consideration so they don’t hurt ur feelings: cancer, pisces