idxstats option for samtools

[GenoMax]

I searched for previous posts on this but the question below was not explicitly answered in the three threads I found.

So here goes:

The output from the "idxstats" command in samtools generates four values. What exactly does the last column (# unmapped reads) signify?

Is "idxstats" the best way to "count" mapped reads in BAM files?

Thanks in advance.