Left-align and normalize indels; check if REF alleles match the reference; split multiallelic sites into multiple rows; recover multiallelics from multiple rows.
Regions can be specified in a VCF, BED, or tab-delimited file (the default). The columns of the tab-delimited file are: CHROM, POS, and, optionally, POS_TO, where positions are 1-based and inclusive. Uncompressed files are stored in memory, while bgzip-compressed and tabix-indexed region files are streamed. Note that sequence names must match exactly, "chr20" is not the same as "20". Also note that chromosome ordering in FILE will be respected, the VCF will be processed in the order in which chromosomes first appear in FILE. However, within chromosomes, the VCF will always be processed in ascending genomic coordinate order no matter what order they appear in FILE. Note that overlapping regions in FILE can result in duplicated out of order positions in the output. This option requires indexed VCF/BCF files.
Similar to regions, but the next position is accessed by streaming the whole VCF/BCF rather than using the tbi/csi index. Both regions and targets options can be applied simultaneously: regions uses the index to jump to a region and targets discards positions which are not in the targets. Unlike regions, targets can be prefixed with "^" to request logical complement. For example, "^X,Y,MT" indicates that sequences X, Y and MT should be skipped. Yet another difference between the two is that regions checks both start and end positions of indels, whereas targets checks start positions only.
For the bcftools call command, with the option -C alleles, third column of the targets file must be comma-separated list of alleles, starting with the reference allele. Note that the file must be compressed and index. Such a file can be easily created from a VCF using:
bcftools query -f'%CHROM\t%POS\t%REF,%ALT\n' file.vcf | bgzip -c > als.tsv.gz && tabix -s1 -b2 -e2 als.tsv.gz