what is Group_Mark_Id '-1' in mtl_serial_numbers

We are on 12.1.3 and have some serial numbers in mtl_serial_numbers table with group_mark_id = -1 .What does  -1 denote?

line_mark_id and lot_line_mark_id are null but since group_mark_id = -1 we are unable to use the serial numbers on any transactions.

There is a data fix that Oracle provides that updates mark_id fields to null ito make them useable but the script does not update if the group_mark_id = -1 so we would like to know what does -1 denote

Thanks

Rahul