Depth of informative coverage for each sample.
FORMAT=<ID=DP,Number=1,Type=Integer,Description="Approximate read depth (reads with MQ=255 or with bad mates are filtered)">
This annotation is similar to the sample-level DP annotation, which counts read depth after general filtering, but with an extra layer of stringency. Its purpose is to provide the count of reads that are actually considered informative by HaplotypeCaller (HC), using pre-read likelihoods that are produced internally by HC.
In this context, an informative read is defined as one that allows the allele it carries to be easily distinguished. In contrast, a read might be considered uninformative if, for example, it only partially overlaps a short tandem repeat and it is not clear whether the read contains the reference allele or an extra repeat.
See the method documentation on using coverage information for important interpretation details.