The mbox3rdPartyID is your company's visitor ID, such as the membership ID for your company'.
When a visitor logs in to a company's site, the company typically creates an ID that is tied to the visitor's account.
When a visitor accesses a page on which Target is enabled, that visitor is assigned a Target PCID. If the visitor then logs in, and the implementation passes the mbox3rdPartyID to Target, Target connects that visitor's mbox3rdPartyID with the Target PCID.
A different value of visitor Id in my opinion will skew the reporting.
Below documentation might help:
Real-Time Profile Syncing for mbox3rdPartyID