A lot of Blu-ray discs come with a book marking facility. Such facilities have appeared over the years on a number of DVD players. Most were utterly useless, since the moment you popped the disc out your bookmarks were wiped. However my first DVD player, the Sony DVP-S725, had a persistent bookmark feature. It maintained a bit of non-volatile memory in which it kept the status of the last two hundred discs that you played, including any bookmarks you cared to insert.
That was at the hardware level.
Blu-ray bookmarks at done at the software level. Of the 113 Blu-ray discs I have examined exhaustively, 28 have a bookmark facility, and 85 do not. Although the implementation is in software, persistent storage built into the Blu-ray player is used. Even grace period players are required to have 64kB of this. So if you save a bookmark on a suitably equipped Blu-ray disc, you will be able to come back to that point weeks or months into the future, despite having played other discs in the meantime.
But it comes at a cost. Because it is implemented in software, the player needs to load in a chunk of BD-Java programming code as it’s starting up. I’m not sure if this is necessarily the case in theory, but in practice Blu-ray players are incapable of restarting from the same place in a movie as where they were stopped if BD-Java code is loaded. Instead, all such discs must go through the entire boot up sequence again, including trailers, copyright warnings and so on.
An easy restart from where you stopped is nice if you have a remote control that allows you to accidentally stop a movie too easily. Hit ‘Play’, and you’re back into the action within a few seconds. But that restart point isn’t retained over time.
I think I prefer bookmarks, but it doesn’t come without cost.