Block Flags

Seven flags exist so far, one per way a block or an entity can be touched. Each takes the Allowed field described in Region Flags, and each accepts a variable there.

• Region Block Break Modifier: whether blocks inside the region may be broken.
• Region Block Place Modifier: whether blocks may be placed inside the region.
• Region Block Interact Modifier: whether blocks inside the region may be used: chests, doors, buttons, levers, and anything else a right click opens or toggles. Pressure plates and tripwires count as well, whether a player steps on one or an arrow, a dropped item or a wandering mob sets it off, since a room whose levers are locked is not locked while its plates still fire.
• Region Entity Interact Modifier: whether entities inside the region may be used: taking the armour off an armour stand, rotating an item frame, opening a chest minecart, and every other right click on an entity, so trading with a villager, mounting a horse, feeding an animal and putting a lead on one stop as well. Block Interact never sees an entity, which is why a museum that only denies it still has its frames turned and its stands stripped.
• Region Bucket Modifier: whether buckets may be emptied or filled inside the region: lava, water, milk and powder snow alike. Emptying a bucket is not placing a block, and filling one is not breaking one, so Block Place and Block Break never see a bucket at all; without this flag a player denied both could still pour lava into a fully protected region. Scooping a fish, an axolotl or a tadpole is a bucket fill too, so this is the flag that keeps an aquarium's inhabitants where they are.
• Region Trample Modifier: whether farmland and turtle eggs inside the region may be trampled by walking or jumping on them. Those two blocks are left out of Block Interact on purpose, so a farm can be protected from boots without locking every door around it.
• Region Ignite Modifier: whether a player may light a fire inside the region with flint and steel, a fire charge, a flaming arrow or an end crystal. This is the only flag that covers lighting a fire on purpose: Block Interact refuses the block that was clicked, and flint and steel is not a block, so it goes on working on a locked wall. Fire arriving on its own stays the Region Fire Spread Modifier's business.

Which block the flag decides for

A block belongs to a region when its center is inside. A boundary that cuts through a block, easy to get with a polygon wall or a rotated region, can leave a block's corner outside while most of the block is visibly covered. The center matches what a builder sees, so a floor the region covers is protected block for block along the actual boundary line.

All seven resolve at the target's location, never the player's. A player camped just outside a protected region cannot reach in and act on anything inside it by aiming across the boundary. Where the target is an entity rather than a block, an item frame or a bucketed fish, the entity's own position decides in exactly the same way.

The world's own changes count too

A protected place stays the way it was built, and half of what changes a build has nobody behind it. So three of the block flags also decide about the world acting on itself:

• Region Block Place Modifier covers a block appearing where none was: crops and saplings growing, grass, vines, kelp, bamboo and sculk spreading, ice and snow forming, a tree grown outside branching in, bone meal being used, and a falling block landing.
• Region Block Break Modifier covers a block disappearing on its own: ice and snow melting, farmland drying out, and leaves decaying once their logs are gone. Every one of those can be provoked from outside the region, by placing a torch or by cutting the tree the canopy hangs from.
• Region Block Interact Modifier covers a hopper or dropper moving items into or out of a container inside the region, which is what locking that container is for.

 A no build region freezes farms

Because growth is a Block Place decision, a region carrying Block Place denied stops wheat maturing, saplings growing and bone meal working inside it. That is what protecting a city center should do, and it is not what you want over a farm. Give the farm its own region with Block Place Allowed on and a higher priority than the region around it, exactly like the arena inside the city.

A tree growing and bone meal spreading are all or nothing: Minecraft offers the whole set of blocks in one event, so a sapling whose canopy would reach a single protected block does not grow at all, rather than growing a clipped version of itself.

Binding one of these flags' Allowed to a variable changes what they cover, since the world's own changes have no player for a fact to read. Limits and Gaps explains what happens then.