The <condition> part of a rule, in which you could put values like "only spawn if adjoining block exists" or "only spawn if block below exists".
Mostly used in conjunction with wall torches, signs etc which relied on blocks other than themselves - but the new IBlockStates make that irrelevant. AFAIK blocks should no longer calculate their own status using neighbour blocks. (Pistons maybe?!)
Okay, but the "ruined" part of Ruins (or the randomization thereof) pretty much depended upon those sorts of relationships. I could have, say, a crumbling wall structure, and randomly take chunks out of it, but in order to make sure that doesn't result in orphaned blocks floating in mid-air, rule #1 was handy for making sure none of those blocks would be placed unless they were supported by something other than air.
I can still make "ruined" buildings, it seems, but I either make it a static structure (it is ruined in EXACTLY this particular way), or else I just have a few select blocks that might or might not be missing here or there, but there will still be an underlying "skeleton" that will still be intact, to minimize the chance of gravity-defying segments floating about. It's doable, I hope -- just different.
Anyway, I'm presently working my way through my library of structures and modifying them: removing all "random" elements, converting dungeon chests back to plain empty minecraft:chest objects, disabling any RUINSTRIGGER Command Blocks, zeroing out "embed_into_distance" settings -- and then to replace all instances of "preserveBlock" with a placeholder block, such as stained glass). For now, anything of mine that used Command Blocks to spawn custom one-time monsters (or structures) isn't worth the trouble, since that part isn't working. I'll try experimenting with the adjoining templates, to see if they work.
I'm curious to see whether any of my customized Mob Spawners might actually work (if they'll be picked up by the parseruin routine); no harm in trying, I suppose.
My plan is to spawn them all in a superflat 1.12 world to serve as a conversion "workshop" -- one layer of bedrock, one layer of diamond block, and I ought to have a good starting point for base plates to chip away at as needed for parsing in 1.13.
I guess I'll try converting over the old Ruins-bundle ruins as well.