Hi bartnv,

these are Millitary Load Class signs:

https://de.wikipedia.org/wiki/Milit%C3%A4rische_Lastenklasse

and we have tagging for that, though I don't know if StreetComplete will understand once it's tagged:

https://wiki.openstreetmap.org/wiki/MLC

If there are no "standard" traffic signs, you can tag maxweight=unsigned

https://wiki.openstreetmap.org/wiki/Key:maxweight#Vehicle_weight