The current frequency is expressed in field 310 and in fixed field 008/18 "FREQ." The regularity of issuance is expressed in fixed field 008/19. CONSER practice is to record the frequency using the terminology as provided in the Fixed-Length field list of frequency designations unless the stated or actual frequency does not appear in the list.
Take information about the frequency from any source (RDA 2.17.12.2), either from information found in the piece or supplied by the publisher or from evidence of issues already published. If there is no stated frequency, give the probable frequency when there is sufficient evidence to indicate the publishing pattern. Do not use a question mark. If the frequency is unknown, omit field 310 and code the fixed field bytes as "u/u". (CCM: 13.3.2)