Layering probably just means where shards would split on player # in the whole battlegroup, each shard has a server identity (your character name suffix for example) that can be parsed and then each one of those individual shards get layered on top of each other in the same zone. That simulates the old “classic” server pop.
Really though what it means is that classic will have all the same bugs retail has with regards to sharding. Things being there and then gone in real time. That annoying one. Characters being dismounted from passenger vehicles is also a sharding bug iirc.