They're mostly the same, with different answers to tricky questions. e.g. if I stack filesystems A, B, C, and have the same file /foo/bar in all of the layers,

They're mostly the same, with different answers to tricky questions. e.g. if I stack filesystems A, B, C, and have the same file /foo/bar in all of the layers, and then do rm /foo/bar, what happens: 1. Does /foo/bar get removed from the topmost layer, exposing the one below? 2. Does /foo/bar get removed from all three layers? 3. Does /foo/bar get replaced with a "tombstone" record to pretend that it was deleted, while still appearing in some of A, B, or C on its own? These semantics are tricky to get right, and during the process of upstreaming unionfs to the kernel, they made some incompatible changes to the model and chose different answers for these questions, and as a result, renamed it overlayfs.