020: International Standard Book Number (ISBN) assigned to a monographic publication by designated agencies in each country participating in the program.
LC Documentation

Check 020 $a; remove dashes and blanks; discard numbers that are not 10 or 13 digits in length.

Retain only one copy of any number (ie dedupe within the record).

Min.Len=10, Max.Len=13

Matching stops if the number of records retrieved exceeds 'Max Numeric Hits'