Splits commonly occur at the ends of boards or flitches. It looks like they are saying they will measure the average lengths and widths of the splits and deduct these dimensions from the total dimensions of the timber purchased.

From my personal glossary: auf Maß schneiden - to cut to (finished) size

unbesäumte Stammware is unedged timber - also referred to as flitches

Flitch. A portion of a log sawn on two or more faces—commonly
on opposite faces leaving two waney edges.

http://www.fpl.fs.fed.us/documnts/fplgtr/fplgtr113/gloss.pdf...


Bei unbesäumter Ware können Risse im Maß abgerechnet werden."