Whereas the define type behavior captures all objects that the page has access to, the logic type behavior captures the logical expressions used on the page. Such usage involves the visibility or enable statuses of segments or elements. These logical expressions could be explicitly stated in the attributes of the descriptors without separate behavior definitions. There are some useful purposes for these definitions. One primary reason is that, in the early part of the specification, it could be known that a segment would be displayed when the user has outstanding debt, without knowing, at that point, what expression would determine this. An entry in the logic type behavior descriptor would capture this with descriptive ID like user_has_debt; this would then be expanded on subsequently with the rest of the entries. The definition in one place with expressive identifier that could be used in many places also has its advantage.

The other reason is that logical expressions could be cryptic and platform dependent. For example, a check for empty string could look like this {theString == null || theString.lenth()= 0}. dWebSpec defines a minimum set of logical operations, in plain English. At implementation, or pre-implementation time, these could be converted to platform specific terms.