Expression compendia in M3D are versioned to indicate the underlying mysql schema of the database and to denote the particular set of expression data (e.g. E_coli_v4_Build_6 uses mysql schema version 4 and is the sixth compendium built for E. coli). Builds are maintained in perpetuity, allowing researchers to specify the exact dataset used for a particular analysis.
Faith JJ, Driscoll ME, Fusaro VA, Cosgrove EJ, Hayete B, Juhn FS, Schneider SJ, and Gardner TS. Many Microbe Microarrays Database: uniformly normalized Affymetrix compendia with structured experimental metadata. Nucleic Acids Research 2008