For permanent residence, regardless of what category, the final step will be either Adjustment of Status or an interview at the US consulate. There are very specific rules which apply to each option. In fact, the rules are so strict there really is no “option” at all; the law determines which process will be used in a particular case.
If the applicant for an immigrant visa/permanent residence is in the US, he or she may be able to adjust status (AOS) and without having to go home to the US consulate. However, all AOS applicants must have a valid immigrant petition under one of the immigrant family or employment categories. That means an approved I-130 petition of it is a family case or an approved I-40 if it is an employment case. And the “quota” must be current for that case if there is a waiting list for that category and country of birth.
AOS require the person to have been in valid immigration status at all times since the very first entry. If there has been a violation in the past, such as an F-1 student who worked without authorization, there may be a “cure” for the violation if the immigrant visa is based on employment but not for family cases. For family case there may be an exception of the petitioner is a US citizen and the beneficiary is the spouse, parent or unmarried child under 21. As you can see the rules for AOS are very strict and must be carefully analyzed before anything is filed.
If the person is out of the US, and the immigrant petition is approved and the quota is current, the last step of the case is called “consular processing”. This means the immigrant visa and interview will be processed at the US consulate, usually in the country of citizenship. Qualifying for consular processing is the same as AOS except the person is outside the US.
Please note that for very complicated reasons, including the “bars” to reentry, if a person is in the US and not eligible for AOS because he or she is out of status, consular processing is usually not an alternative. It is best to think of using consular processing only for cases where the applicant has not been to the US and is now in his or her home country. For this reason, consular processing is really only used for family preference cases. And, again, a Board Certified immigration attorney should be consulted before even the immigrant petition is filed.