The SPASE metadata model can be expressed in many forms. One of those forms is as an XML Schema. An XML Schema is a machine readable form (see schema). Here you can find human readable documentation which includes standard model diagrams.
Search the model dictionary.
When referencing the Base Model in publications use the DOI: https://doi.org/10.48322/E72C-5Y75
There is a draft version of the data model which includes definitions under consideration for the next production release.