Relationships are affected by constraints. A constraint is something that affects, often negatively, the quality of a relationship.
Controls and rules are two examples of constraints on relationships. A control restricts behavior to ensure adherence to a rule. An example is the required approval of an inspector before a deliverable can be available to someone else. Rules are a subset of controls that require compliance prior to taking action, e.g., complying with certain quality standards of a deliverable prior to release.
Resources can also be a constraint, both from qualitative and quantitative perspectives. They can be quantitative in that enough may not be available. They can be qualitative because they may lack the requisite abilities to accomplish specific tasks . Of course, constraints can be quantitative and qualitative. An example of a quantitative and qualitative constraint is people on a project. There might not be enough people with a specific skill and who may lack the ability and willingness to do the necessary work.
Not all constraints, like relationships, are apparent. An apparent constraint is cost and schedule. Many constraints in a system are implicit, which makes them very difficult to discern. On a project, a frequent example is whether or not a team member can perform specific tasks. Despite having a good knowledge of people's capabilities, it is very difficult to determine the degree to which they can actually perform tasks. Implicit factors might include a willingness and ability to perform a task. It is only through time and experience with an individual that project managers can really determine the degree to which people are a constraint. Often, project managers have no other choice but to accept a constraint in this regard.
This last example raises an important point about systems. According to systems theory, all systems possess "energy" to reach their goals. Unused energy is latent or potential energy; used energy is realized or kinetic energy. How this energy is tapped determines the degree to which a system performs efficiently and effectively. The measure of how well the energy is used is reflected in the quantity and quality of output. A good example is the use of the cost performance index and schedule performance index to measure efficient usage of realized energy on projects; defect rate is a good measure of the effectiveness of the realized energy of projects.