[gentoo-user] mkvtoolnix: To use qt5 or not to use qt5 - that is the question (USE flag in-/valid?)

Hi

this command:
emerge --selective=n  @preserved-rebuild

gives me this message when started:
--- Invalid atom in /etc/portage/package.use/mkvtoolnix: qt5

and eix said this:
[I] media-video/mkvtoolnix
     Available versions:  37.0.0^t (~)42.0.0^t (~)43.0.0^t **9999*l^t {debug nls pch qt5 test}
     Installed versions:  43.0.0^t(06:02:33 PM 02/23/2020)(nls -debug -pch -qt5 -test)
     Homepage:            https://mkvtoolnix.download/ https://gitlab.com/mbunkus/mkvtoolnix
     Description:         Tools to create, alter, and inspect Matroska files

From this I cannot decide, how to handle the 'qt5'-flag:
Is it valid or is it invalid (and how do I get the gui version of that
tool...) ?

Cheers!
mcc