29 CFR § 29.4 - Criteria for apprenticeable occupations.
An apprenticeable occupation is one which is specified by industry and which must:
(a) Involve skills that are customarily learned in a practical way through a structured, systematic program of on-the-job supervised learning;
(b) Be clearly identified and commonly recognized throughout an industry;
(c) Involve the progressive attainment of manual, mechanical or technical skills and knowledge which, in accordance with the industry standard for the occupation, would require the completion of at least 2,000 hours of on-the-job learning to attain; and
(d) Require related instruction to supplement the on-the-job learning.