Complex sentence: In library science, the number of pages in a book forms part of its physical description, coded in subfield 300 $a in MARC 21 and in subfield 215 $a in UNIMARC.
Split into simpler sentences: In library science, the number of pages in a book forms part of its physical description. The numbering of pages is coded in subfield 300 $a in MARC 21 and in subfield 215 $a in UNIMARC.