CORE, FLEX AND PAN GENOME. “Early bacterial whole-genome sequencing efforts discovered that whenever a particular “species” was re-sequenced, new genes were found that had not been detected earlier—entirely new genes, not merely new alleles. This led to the concepts of the bacterial core-genome, the set of genes found in all members of a particular “species”, and the flex-genome, the set of genes found in some, but not all members of the “species”. Together these make up the species’ pan-genome.”
https://standardsingenomics.biomedcentral.com/articles/10.1186/s40793-016-0180-8