A Systems Integration Test (SIT) is what engineers do when they're too old to play with meccano sets.
Large engineering projects often involve many different companies working over a very distributed area, commonly including several countries or continents. Even within one company different divisions often work independently from each other. When complete, each of these different project components needs to work in harmony with the parts being produced elsewhere by other people. A likelihood of which is in inverse proportion to the complexity of the project and number of companies involved.
Enter the Systems Integration Test, performed to ensure that all of the components fit together correctly and as specified. This means getting all the pieces to the same place, and then performing a full functional test. In the case of a mechanical engineering project it means physically putting all the pieces together, connecting all the joiners, inserting plugs into all the receptacle, turning all the levers, and so on.
SITs are used extensively in the offshore industry, where offshore costs are orders of magnitude greater than onshore costs. An SIT allows an operator to confirm that the remotely operated vehicle contracted to do the job (and its associated tools) will be physically capable of performing the required tasks once in the field. With offshore per diem often exceeding $20k it is very important to know no time is going to be wasted.
An offshore SIT means collecting the ROV, ROV pilots, and subsea asset (wellhead, manifold, etc.) into a large warehouse. The ROV is then hoisted onto a crane (to simulate three degrees of freedom -- like being underwater) and held in front of the asset. The pilots and engineers will then step through the workscope doing the same job they will be doing in anger once offshore. This allows you to find the optimal way of doing the job and test for problems in the workscope. It also means you can test to ensure the ROV seven function manipulator can reach what it needs to and confirm that the pilots line of sight is never obstructed, as well as ensuring any plugs or tools will fit into the receptacles they were built for. At the same time, the engineers can weld any required grab points to the asset (in the process destroying the carefully specified anti-marine growth paint coat) should they be required.
Though most commonly used in the mechanical and electrical engineering fields, the same principles are now also used in fields like software development to ensure components created separately fit together as specified.