If you do, they don’t get cleaned up during the normal ‘hammer cleanup’ nightly routine. Chris Turner has added a way to manually specify them as a cleanup target.
I’m pretty sure in this case ‘offline’ means ‘nothing streaming to it from a master disk’. I think.
I think what Chris means by “offline” is a pfs that’s not mounted. I’m happy about this change because I have a few pfs that don’t need to be mounted but had to be mounted anyway just so that the nightly hammer cleanup would see them.