Not to be confused with restraint which is much more about self-control, constraint is about finding the edges of the page before you begin, it is about knowing what limits you have in terms of resources. It is about must haves and must nots. And to be honest not something I previously worried too much about, but now I see constraint in lots of work that I do and inevitably seek them out if they are not so explicit. source
Tom Barrett explores various examples of constraints and the creative possibilities that it provides. This is not to be confused with restraint, those aspects impose self-control. Constraints come in many different forms, including space and design.