A MPID, also known as a mParticle ID, is a 64-bit signed integer that is used by mParticle to uniquely identity a user for the purposes of processing User Identities and Profile data.
While your implementation assigns User Identities such as Customer ID, Email, etc., MPIDs are automatically posted by mParticle's backend systems to resolve identity requests.
All users, whether anonymous or known, will be assigned an mPID for the purposes of retrieving profile data. When an anonymous user provides identifying information such as Email, mParticle will assign a new MPID and create a profile according to your account's Profile Conversion Strategy.
You can retrieve a user's information by searching using their mPID in User Activity View
Learn more about an MPID in our docs