Hi Lincoln
As far as I can tell the primary inspection is where an officer checks your ID/passport and makes an initial assessment of whether everything checks out and you're not suspicious. If this is the case off you go. It's the booth where you show your passport, car registration, etc. If there is something in your papers that doesn't convince the officer or they are in any way suspicious you might be a mule, or other type of criminal, illegal entry, etc., you'll be taken to a secondary inspection area, which is presumably where they can strip search, X-ray people, take their cars/luggage apart, hold them for a period of time, things like that.


See
Privacy Impact AssessmentU.S. Customs and Border Protection, TECS Page 4 history, including previously issued I-94 or I-94W arrival/departure records in the Nonimmigrant Inspection System (NIIS), recorded prior violations of law, and records in the Seized Asset and Case Tracking System (SEACATS). Upon arrival in the U.S. in the air/sea environment, individuals are generally required to present themselves to CBP at the POE’s primary arrival location (primary). At primary, CBP obtains information directly from the traveler via his or her presented travel documents (e.g., passport) and/or verbal communication between the CBP officer and the traveler.7For all individuals entering the U.S. at a POE, a record detailing the traveler’s border crossing is captured through TECS and maintained in the Border Crossing Information System (BCI). The information collected in primary is matched against the previously provided APIS information and any Subject Records, to the extent they exist.. At this point, CBP will collect any required forms such as, the I-94 Arrival Departure Record or FinCen 105 Currency/ Monetary Instrument Report (CMIR) (a Bank Secrecy Act report concerning the international movement of more than $10,000 worth of currency). These collections, in the air/sea environment, are not an event that automatically necessitates referring a person to the secondary arrival location (“secondary”). If the CBP officer at primary determines that additional inspection is needed, the traveler will be referred to secondary. A record of the referral and secondary inspection is entered into TECS as a Subject Record. During a secondary inspection, a CBP officer may run law enforcement queries through TECS and other systems on the TECS Platform. Regardless of the outcome of the secondary inspection, the CBP officer will create a record of the secondary inspection in TECS. If the secondary inspection results in a violation being discovered, then a record may also be created in SEACATS.
https://www.dhs.gov/xlibrary/assets/privacy/privacy-pia-cbp-...

https://www.google.com/search?client=firefox-b-d&q=Primary I...