Project constraints are anything that restricts or dictates the actions of the project team. That can cover a lot of territory. The triple constraints—time, resources, and quality - are the big hitters, and every project has one or two, if not all three, of the triple constraints as a project driver. Many projects in the Information Technology area, for instance, are driven by time. Projects in the pharmaceutical industry are driven by quality but may have time or resources as a secondary constraint.
For example, a business relocation project would likely primarily be constrained by time. The secondary constraint for such a project would budget - there is a limit to how much can be spent. What you want to do now is to document the project constraints. You can basically use the same techniques you use to document project requirements and project assumptions.
Besides the triple constraints, don't overlook constraints like these that can cause problems on your project:
Lack of commitment from the executive management team or project sponsor. I would consider passing on the opportunity to lead a project with this constraint - if it’s an option. You'll have a hard time getting support or resolution of problems. Watch out for this constraint because it can creep up on you later on in the project. The sponsor may lose interest because other things have come along that usurp the priority of this project and so on.
Business interruptions or reorganizations in the midst of the project. This could potentially realign your project resources, leaving you empty-handed.
Stakeholders who have unrealistic expectations of project outcomes. This one is overcome through good project communications and requiring sign-off of the project charter and scope statement documents.
Stakeholders' unrealistic expectations of the project schedule. This is also overcome through good project communications early in the project.
Lack of skilled resources. This could cause project delays or unfilled deliverables. And customer satisfaction could take a huge hit on this one as well as their anxiety mounts throughout the engagement.
Poor communications. This is a potential project killer. Misunderstandings regarding scope, activity assignments, project schedules, risks, or a long list of other project essentials could cause uncorrectable problems.
Uncertain economic times or business conditions. Difficulty obtaining funding for projects, resources for projects, and general economic disturbances could restrict the project team.
Technology. Advances in technology can cause project delays due to lack of knowledge of the new technology, training needs or availability of training, availability of resources with experience in the new technology, and so on.
Constraints, like assumptions, are also documented in the scope statement. These should be updated as you progress through the project to make adjustments to the constraints you've listed, add new ones that may come up along the way, or delete those that are no longer a constraint. Remember that sometimes you'll find that constraints are also project risks and may need risk response plans.