The chemical structure shown (which is analogous to those used in KEGG and MetaCyc) is a generic structure devised to represent a common steroid skeleton. Derivatives with ring substituents at positions other than positions 3 and 16 as well as those containing double bonds, aromatic A-rings, expanded/contracted rings etc. are also included.
Although this definition of sterols is rather restrictive, a less restrictive definition would cause difficulties, e.g. androst-16-en-3β-ol is normally not called a sterol and should not fall under the definition.