Rule 10204 of L2 V4 r1 says:
In the SBML subset of MathML 2.0, the MathML attribute definitionURL is only permitted on
csymbol and semantics. No other MathML elements may have a definitionURL attribute. (Refer-
ences: L2V2 Section 3.5.1; L2V3 Section 3.4.1; L2V4 Section 3.4.1.)
Section 3.4.1 says the same.
DefinitionURL should be allowed on ci too, so people can use SBO. We made the change in L3V1. We should make it in L2V5 too.