852 $8 - Link and sequence number:

Subfield $8 contains data that sequences related holdings records. The linking number is a variable-length whole number which controls the display and processing of multiple related holdings records that contain identical 852 subfields $a, $b and $t. Local systems may add decimal numbers if desired for insertions in an existing sequence. This subfield does not order fields within a record, but locations in separate holdings records. Example:

Subfield $8 (Link and sequence number) is always the first subfield in the field.