Automatic constraint conversion makes joints out of certain kinds of constraints. Manual constraint conversion does much the same, but gives you control over which constraints are converted. Even though certain constraints are automatically or manually converted, that process may not provide all of the joints required by your simulation. To complete your simulation you can insert joints manually. You can choose from a variety of joints such as spatial, rolling, sliding, 2D or 3D contact, and so on.
In addition to adding joints manually, you can use joint options and parameters to modify a joint so the simulation reflects the behavior of your product. After doing so, you can test and verify that the joints function as intended.