The subject fields are important points of availability and access in bibliographic recordings. They are an inseparable part of the mandatory fields of registration. The study examined the substantive fields of availability in Mark 21's bibliographic registration of topic heads and classification numbers in terms of their identification, indicators and subfield details, as well as identification of the rules and criteria governing these fields in the recording.The study relied on the descriptive approach of describing the phenomenon studied by reference to the subject's literature s bibliographic record is not limited to a single field number, but the ranges of those fields vary according to the approved classification line or classification system, which also applies to the fields of the topics head.