Okay, so here I am planning to add a hypothetical Maker's Quarter.

I see that the district itself will move production just from 2 to 3, and that the exploitation to the east will add 3 more production.

I see that the district itself would wipe out 1 food.

But how about that tile to the northeast?  Why is it not also now an exploitation tile, adding food?

I must be missing some simple concept, but I cannot see what it is.

Ruins?

Not as far as I can tell.  Seems to be just a Prairie River +3 food.

Turns out that, if in the first image, I try to place a Farmers Quarter, then the northeast tile is considered an exploit tile.  Weird.

So glad I asked.  Thanks.

My memory may be off, but as I remember it, in Endless Legend when you put down a district all the new adjacent tiles were exploited, whether they "matched" or not.  

In any case, this is pretty important to know.  :)