This notation is used for example to retrieve data from the NIST Atomic Spectrum Database.
[4] The spectroscopic notation of molecules uses Greek letters to represent the modulus of the orbital angular momentum along the internuclear axis.
For Σ states, one denotes if there is a reflection in a plane containing the nuclei (symmetric), using the + above.
For homonuclear diatomic molecules, the index g or u denotes the existence of a center of symmetry (or inversion center) and indicates the symmetry of the vibronic wave function with respect to the point-group inversion operation i. Vibronic states that are symmetric with respect to i are denoted g for gerade (German for "even"), and unsymmetric states are denoted u for ungerade (German for "odd").
For mesons whose constituents are a heavy quark and its own antiquark (quarkonium) the same notation applies as for atomic states.