Indicate how often a given read group occurs in genome
Currently, all compatible alignments are used and counted whole (i.e., with 1) towards a sequence. However, this can be misleading if the reads of a given read group are align equally well to other genomic loci ("multimappers").
Indicating, for each read group, the number of genomic loci that that read group equally well aligns to will help interpret whether a given read group really originates from the locus in question.