I was trying to figure out what this was but from the limited amount of info I found I found out that 100 is the nominal setting for a 10 megabit link. What should this be set for a 30 megabit link or 50, or 100? What about something like 50/10 that isn’t symetrical?
It is used for the spanning tree protocol. When you create a bridge there is a possibility that a layer 2 loop is created with devastating consequences. Spanning tree protocol detects such loops and if necessary disables a link to prevent them. The value is a manual way to set a preference which link to be shut first in case of a loop. The higher value indicates a link more probable to get shut down.