Rather than being set explicitly, the values of some types of attribute can be specified in terms of rules; that is, expressions from which the attribute values can be evaluated. Rules can be set only for attributes of the following types (including user-defined attributes): text, scalar (integer, real or logical), position, orientation, direction; they cannot be set for reference attributes. A static rule will change the attribute setting only when verified or executed explicitly, whereas a dynamic rule will update the attribute setting whenever any part of the expression changes (the default type is static).