The standard BOSS analysis pipeline (included in idlspec2d) provides a set of spectroscopic parameters for each 1D spectrum, that are determined via Principal Component Analysis (PCA) model fitting of quasar, galaxy, and stellar templates. This process provides the best fit redshift and classification (both with and without including quasar templates) for each spectrum. The template fitting is supplemented by routines that make emission line measurements and estimate stellar properties from the 1D spectra. The SPEC1D analysis pipeline is described on the BOSS Pipeline page. The best fit BOSS spectroscopic parameters are provided in the spAll summary file, and the spAll extension of 1D spectra files (specFull/specLite), and are described by their associated datamodels.
In some cases, multiple models provide similar quality fits. These can be identified by the ZWARNING flag of SMALL_DELTA_CHI2. The parameters associated with these alternative model fits can be found in the ZALL extension of the spectra files (specFull/specLite) or via the spZall files, as described in the BOSS Alternative Redshifts tutorial.
A set of spectroscopic parameters are computed from each coadded spectrum (See this page for a description of the available BOSS coadds). Therefore, in general, we provide multiple sets of spectroscopic parameters computed per astrophysical object. It is left to the science user to determine which is most relevant for their project.
Before SDSS-V, the stellar parameters were solely determined using the stellar Elodie models, but in DR19, these were suppressed from the summary files in favor of the parameters determined by pyXCSAO. All spectra are processed by pyXCSAO, but these parameters are invalid for non-stellar sources and should be ignored.
Outside of the standard BOSS pipeline, additional parameters can be found in value added catalogs, or Astra (for MWM stellar source).