euse does not recognize USE-flags defined by in overlays.
Instead euse gives an ERROR saying that the USE-flag does not exist.
The overlays are configured using the PORTDIR_OVERLAY variable in make.conf.
'man euse' explicitly mentions both $PORTDIR/profiles/use.desc and $PORTDIR/profiles/use.local.desc, so looks like it's intentional limit. Just like 'equery uses'.