what is "set merit " in media player classic?

[Mohammad48]

what does "set merit" do? what does the number mean?

you can see it in options > external filters

[vivan]

When the Filter Graph Manager builds a graph, it enumerates all the filters registered with the correct media type. Then it tries them in order of merit, from highest to lowest.By default filter with higher merit will be used (if internal filters are turned off). That could be overrided with "prefer".
That matters more for other players that don't have inernal filers and "prefer" system, like WMP or something.