I read the devmanual regarding IUSE variable and I did not understand the meaning of + and - prefixed USE flags in IUSE, since that's not mentioned there. It's described in ebuild(5). Page in question: https://devmanual.gentoo.org/ebuild-writing/variables/index.html#iuse This information is available in ebuild(5) so I recommend that a link is added to this devmanual page.
It is described here: https://devmanual.gentoo.org/general-concepts/use-flags/index.html#iuse-defaults
Thanks, sorry for the disturbance, this seems to be a non-issue then.