Not all employment based categories require labor certification. None of the sub-categories in EB1(Employment based first preference) required labor certification. In EB2(Employment based second preference), labor certification is required unless it can be proved that the alien's work in the national interest of United States and hence labor certification should be waived (It is called National Interest Waiver(NIW)). All sub-categories in EB3(Employment based third preference) required labor certification.

EB4 (Employment based fourth preference) created for ministers and religious workers, replaces the labor certification requirement with a requirement of demonstrated affiliation with the employer. The value of a minister or religious worker is arguably unquantifiable, and a position with a church or other religious organization, at least in the abstract, is not appropriate for labor certification.

EB5(Employment based fifth preference). It is actually not employment-based but rather an employment-creating one. EB-5 is reserved for would-be immigrants who invest $1 million (or $500,000 in rural or high-unemployment areas) to create ten or more jobs for U.S. workers. So there is nothing certify. So labor certification is required.

Certain low-skill occupations are considered inappropriate for labor certification because the U.S. Department of Labor has determined that there generally are sufficient United States workers who are able, willing, qualified and available for the occupations.

Certain occupations are pre-certified due to a lack of qualified U.S. workers:
physical therapists
professional nurses
aliens with exceptional ability in the sciences or arts

Certain aliens are exempted from the labor certification process, i.e., those immigrating on the basis of a family relationship, diversity immigrants, refugees and those granted asylum, even though many will be undertaking employment in the United States.