articles - Pls explain what exactly this "Priority Seat" represents

I would like to understand what exactly this "Priority Seat" represents.

Image credit: Southern Rail, UK

For a long time, I was thinking "Priority Seat" is the name of an adjacent priority seat so that "A" is not required. Just like the name of your iPhones is "iPhone" not "an iPhone," But someone told me that "Priority Seat" is not a proper noun. The words are capitalized because it's a title.

Then I came up with a new explanation, that is, this "Priority Seat" is simply an abbreviated form of "A Priority Seat"; "A" is omitted because it is a sign. But when I imagine a sign that reads "A Priority Seat" in this context (adjacent to a priority seat) I feel so incorrect. The text wouldn't work to create the right nuance. So I had to drop this idea as well.

Could anyone explain what kind of grammatical structure is used for this "Priority Seat" so that I could clearly understand what it represents?

Answer

You should envision the sign saying,

"This is a Priority Seat"

but because it's a sign, it's shortened to just "Priority Seat."

It's the same with signs that say, "Restroom" or "Exit" or "Lobby" or "Parking Garage."

This works because signs like these are labels and labels say what the things they label are.