Minecraft wiki entry for Chunk, associated text: Block changing on the edge of a ticking chunk can spread updates to blocks outside the ticking chunk and make them respond appropriately. The update may be propagated block by block until outside the border chunks, at which time it creates a temporary ticket to continue propagation.
Block in a chunk with a 33 level can request a scheduled tick, but it does not get processed until the chunk gets a load level of 32 or below.
Flowing water or lava can spread to the first adjacent block outside a ticking chunk, but the flow becomes suspended there until the border chunk has a greater load level.
Fire can spread to the first adjacent flammable block outside the ticking chunk. Like water and lava, it becomes suspended there. It cannot spread further until the outside chunk gets a level of 30 or below.
Grass and mycelium can spread to the first adjacent block outsider an entity ticking chunk.
Pumpkin and melon stem growing on the edge of an entity ticking chunk can place their fruits on an adjacent block outside an entity ticking chunk.
An entity (mob, minecart, arrow, etc.) that attempts to move into a ticking chunk from an entity ticking chunk becomes suspended as soon as it leaves the entity ticking chunk. When the ticking chunk gets a greater level, the entity resumes moving.
Exploding TNT in entity ticking chunks can damage or destroy blocks in a non-entity-ticking chunk.