I suspect something got lost in translation. Normally a "length per length" spec for a level would mean that one division on the bubble/scale corresponds to a rise or fall of the 1st length over the 2nd length. But 0.05 M/M would mean if you set the level on a meter stick or 3 foot straightedge and lifted the end of the stick/straightedge 50mm (almost 2 inches), the bubble would only move 1 division. Which is a really crummy precision.
I have sometimes seen foreign specs where M/M is an abbreviation for mm. In this context, I'd interpret that as "over the length of the level, whatever that is, a rise or fall of 0.05mm is one division on the bubble". If that's what it is, that seems like a pretty good precision.