When creating products + combinations using the CSV import tool, references are by default only associated to combinations, leaving the parent product with no reference.
0: references are added to combinations, parent product has no reference [REF01 - ]
1: references are added to combinations, the reference added to the parent product will be the same as its first combination [REF01 - REF01]
2: references are added to combinations, the reference added to the parent product will be the same as its first combination and the letter P [REF01 - REF01P]