The SML spec mentions in section 2.2 Terminology the notion of 'rule documents' :

>>>This specification defines two types of model definition document - Schema documents that conform to XML Schema and rule documents that conform to SML's profile of Schematron....


The following paragraph in section 5 - Rules talks about schematron rules that are defined in their own document, as opposed to attached through an appinfo to an element type. It is not clear that these are the 'rule documents' mentioned in section 2.2 Terminology as they are not even once called 'rule documents'.
This is the paragraph in section 5 Rules that refers to this schematron rules :

>>>Model validators that conform to this specification MUST provide a mechanism to support binding of Schematron patterns that are authored in separate documents, i.e., not embedded in schema definition, to a set of documents in a model. The mechanism for binding such Schematron patterns to a set of documents in a model is implementation-dependent and hence outside the scope of this specification.  The following example shows the constraints for StrictUniversity expressed in a separate document:

Issues :
1. Although 2.2 mentions document rules it does not define the notion. I would expect since this section is about terminology, to state what a rule document is. 

2. Section 5 - Rules does not refer to the schematron rules defined in separate documents as 'rule documents' so there is no way to link this section with the terminology introduced in 2.2. The paragraph from section 5 copied above should be updated to mention 'rule documents' in relation to the schematron rules mentioned there.