A Direct Integration User ID lets you submit your appraisals directly to UCDP using Mercury Network. As a Mercury Network user, you'll definitely need to request your Direct Integration after you've registered for UCDP.
The DI User ID is set up within a specific Business Unit. The most common configuration will be to request the DI User ID within your Primary Business Unit. However, if you have multiple Business Units and intend to submit appraisals to UCDP differently for various Business Units, your configuration may require a little more thought. See more about Business Units here.