Two tiers, and the difference is everything
Immediate relatives of US citizens, meaning spouses, unmarried children under 21, and parents of adult citizens, have a visa available immediately. Everyone else falls into a preference category and waits for a visa number.
The preference categories
These cover adult children of citizens, spouses and unmarried children of permanent residents, married children of citizens, and siblings of adult citizens. Waits vary enormously by category and by country of chargeability, and the monthly Visa Bulletin is where they are tracked.
Age and marital status can move a case
A child turning 21 or a beneficiary marrying can shift a case into a different category with a different wait. There are protections in some circumstances. Both are worth understanding before they happen rather than after.
Where the case finishes
Adjustment of status inside the country if eligible, or consular processing abroad. Which applies depends on how the person entered and on their status history, and it should be assessed before anything is filed.
