An old post but replying because the current answer is incorrect -- the "<*>" allele was added in VCFv4.3; it is a symbolic allele meant to "represent reference evidence as likelihoods against an unknown alternate allele" -- basically, it is the ALT allele given when there are no samples that don't match the reference, but the maker of the VCF still wants to include the locus in the file. It is different than the "*" allele (spanning deletion) which represents the absence of that position in the alternate genomes.