The text serialization of mzSpecLib is somewhat similar to the popular NIST MSP format, but is formally standardized, which MSP never was. Furthermore, it adds important features, such as support for spectral clustering output, support for unidentified (but commonly observed) spectra, support for multiple analytes per spectrum, and alternative hypotheses for the interpretation of each spectrum. It also explicitly makes use of the PSI-MS controlled vocabulary, rather than using somewhat ad hoc keywords in the comment field that were never properly defined. Thus, while the formal mzSpecLib specification is expected to remain fixed for a long time, its use can be sufficiently flexible via the externally maintained controlled vocabulary. This principle is analogous to what has been followed in all PSI data standards.