I often face the problem of missing documentation of MOSS 2007 operations at high level, it is documented how to configure it and the very low level details in protocol specification.
Profile import is using the crawler component of MOSS search. A user profile import is actually a crawl of special content - people.
It is using the same pipeline mechanism with mssearch.exe and mssdmn.exe , the difference is that during profile import the crawler uses a protocol handler written for AD, LDAP connections to handle AD users, groups and AD/LDAP properties.
LDAP and AD connections are called primary connections because these can import new user profiles. BDC connections are called secondary since they can only add supplementary properties to already imported user profiles from custom data source like SQL or other enterprise applications.
The integration can also be noticed when you check the import log which is the same as the crawl log.
Profile import is using two special content sources PEOPLE_IMPORT and PEOPLE_DL_IMPORT and uses with a special start url using spsimport:// prefix.
User profile import process has 5 main steps:
Incremental profile imports are supported when the AD connection account has rights to query the replication changes from the DC.
User Profiles and Audience Targeting Overview
Managing User Profile