Structural complexity of lipid species and sum composition constraints. Let us consider phosphatidylcholines (PC class lipids) as a representative example: PC molecules consist of a posphorylcholine head group attached to the glycerol backbone at the sn-3 position, while fatty acid moieties occupy sn-1 and sn-2 positions (alternatively, a fatty alcohol moiety could be attached at the sn-1 position). Fatty acid moieties differ by the number of carbon atoms and double bonds, but also by the relative location at the glycerol backbone, so that isomeric structures having exactly the same fatty acid moieties are possible. Note that isomeric structures are always isobaric, whereas isobaric molecules are not necessarily isomeric. Most generic constraints ('All lipids of PC class' or 'All PC esters') encompass sum compositions of species with all naturally occurring fatty acids. However, because of the fatty acid variability, some species of other lipid classes (such as phosphatidylethanolamines (PE class)) might meet the same constraint. Therefore, for most common glycerophospholipid classes, the characterization of individual molecular species can not rely solely on their intact masses, irrespective of how accurately they were measured. MS/MS experiments that produce structure-specific ions contribute more specific constraints, such as the number of carbons and double bonds in individual moieties, characteristic head group fragment, characteristic loss of a fatty acid moiety, among others. Within a MFQL query, these constraints can be bundled by boolean operations.