Constraints are a concept that are visible in all 5 core elements of ITIL version 3. Constraints are the boundaries against which solutions can be developed based on customer needs.
There is a wide variety of constraint types; but no definitive list for particular services. Constraints can be financial, cultural, legal or resource based. The constraints of a service have many overlapping intersections. The free area that is boundaried by the constraints is referred to as the solutions space.