The need for Business Rules
The Semantics of Business Vocabulary and Rules (SBVR)  defines business rules as being "a law or principle that operates within a particular sphere of knowledge, describing, or prescribing what is possible or allowable" and that is also under the jurisdiction of a particular business. In the AIXM case, that jurisdiction is the aeronautical information domain.
The AIXM UML model defines the information items that are in the scope of the "aeronautical information" domain using UML class diagrams. This includes definitions for classes, attributes and associations between classes. For attributes, the constraints expressed as lists of values, range of values, pattern are also included in the AIXM UML model. The AIXM XML schema is derived from the UML model and defines elements that correspond to the AIXM model classes, attributes and association role names. The values of XML elements and data types derived from UML attributes are constrained based on the data types defined in the UML model - list of values, data ranges and/or patterns.
More complex constraints, such as dependencies between the values of different attributes (sometimes in different classes), detection of 'out of range' values, mandatory properties for class of objects, etc. are not included in the UML model and do not appear in the XML Schema. Thus the need to document such more complex constraints as "business rules". The aim of providing AIXM Business Rules is to be able to define what is possible or allowed in an AIXM data set, in particular with regard to data values.