I can’t find this documented anywhere.
Is Priority in Firewall->Mangle, range 0-7?
And is the lower the priority number, the higher the priority given?
This page describe the priority numbers pretty well: https://wiki.mikrotik.com/wiki/Manual:WMM
Hope it helps.
To me it looks like any other label. Any string seems to be allowed. I already asked the forum about it:
http://forum.mikrotik.com/t/please-explain-ingress-priority/125576/3
Thank you