Mattia, the paragraphs under 8 CFR 214.2, (a), (b), (c), etc, correspond to the various visa classes established under §§101(a)(15)(A), 101(a)(15)(B), 101(a)(15)(C) of the Immigration and Nationality Act 1952 (as amended). Therefore §214.2 can be limited to paragraph (m).
INA §101(a)(15)(M)(i), establishes the M-1 class of nonimmigrant visa:
(M) (i) an alien [...] who seeks to enter the United States temporarily and solely for the purpose of pursuing a full course of study at an established vocational or other recognized nonacademic institution (other than in a language training program) in the United States [...]
Applicable sub-paragraphs under 8 CFR 214.2(m) are:
(1) Admission of student, (2) Form I–20 ID copy, (3) Admission of the spouse and minor children of an M–1 student, (4) Temporary absence, (5) Period of stay, (6-9) [Reserved], (9) Full course of study, (10) Extension of stay, (11) School transfer, (12) Change in educational objective, (13) Employment, (14) Practical training, (15) Decision on application for extension, permission to transfer to another school, or permission to accept employment for practical training, (16) Reinstatement to student status, (17) Spouse and children of M–1 student, (18) Current name and address, (19) Special rules for certain border commuter students, (20) Remittance of the fee.
Under paragraph (m)(9),
full course of study:
(i) Study at a community college of junior college [...]
(ii) Study at a postsecdonary vocational or business school [...]
(iii) Study in a vocational or other nonacademic curriculum [...]
(iv) Study in a vocational or other nonacademic high school curriculum [...]
(v) On-line courses/distance education programs [...]
(vi) Reduced course load
See
here for formatted version of 8 CFR 214.2(m).
The other paragraph mentioning 18-hours is under paragraph (m)(9)(iv) and is therefore irrelevant.