UK (British) style RNA
There are various styles of RNA (Roman Numeral Analysis). In Britain the notation Xa - or just X - is used for a root position triad., where X is one of I,II,III,IV,V,Vi or VII [or possibly also I,ii,iii,IV,V,vii - VII is a special case - with lower case letters denoting minor chords].
The notation Xb denotes a first inversion triad - with the third in the bass, while Xc denotes a second inversion triad with the fifth in the bass.
The notation is also extended for seventh chords - example G7 = G B D F. Thus V7 [the 7 would probably be a superscript ..] would denote the G7 chord in the key of C, V7b would have the B in the lowest position, V7c would have the D in the lowest position, and V7d would have the F in the lowest position.
For seventh chords the 'd' suffix denotes third inversion.
RNA notation in the US is different, and similar in ways to figured bass notation.
A problem arises with the British system in MuseScore as the 'b' suffix on a chord symbol is interpreted as a flat - and RNA is treated using chord symbol rules. I think it's possible to get the notation without using the b to denote a flat - but I can't see any clear reference to this in the handbook. Some form of escape code is used before the 'b' to have it treated as a plain b, not as a flat symbol.
British style RNA is used in many books and music schools around the world.
It really would be useful to have clear guidance on this and an appropriate reference in the handbook.