From ISOGG Wiki
When the allele(s) for a SNP are not determined during the process of genotyping a person's DNA and allele values (such as A, C, G or T) cannot therefore be reported. The SNP in question gets reported as a no call. The usual notation for this is with either the symbol - or the symbol ?
When comparing two people's autosomal DNA results, it is customary to treat no calls as SNPs that match. That is, they are considered wildcards within most matching algorithms.