瑜伽論記 17.471

17.471

解云：相應緣境有勢用義，故立禪支，戒體非緣不立禪支，但戒八正道中道輪為轂，故立道支。

The explanation says: Having conjunction, taking an object, and having efficacy establishes something as a dhyāna limb; the substance of morality does not take an object, so it is not established as a dhyāna limb. However, in the noble eightfold path, morality serves as the hub of the wheel of the path, and is therefore established as a path limb.