Some genealogy programs use top-level custom records that are specific to their program. They may export these to a GEDCOM file that you import to Behold. Any such records are displayed in the Custom Record Details section. Each type of custom record has their own section and records within the section are numbered sequentially.
The above example is from a GEDCOM produced by the program PC_Ahnen which adds additional information about places in a custom LOC record.
The title of the section is based on the record type used in the genealogy data. In this case the GEDCOM file had a record type of "_LOC", where the leading underscore is the GEDCOM convention for allowing custom tags.
References to each event or fact that links to the custom record are given. They include the event/fact, date, place (as a hyperlink to the place), name of the person (as a hyperlink to the person) and the number of the person.