Two switches on a field definition change how many values it takes.
Allow several values
Turn on Multiple and the field accepts more than one value. Depending on the type it becomes a tag input or a set of checkboxes.
Use it for “this sofa is available in these fabrics”.
Repeat per related item
For Item fields there is a second switch: Repeat per related item. The field then appears once for each thing it is related to.
This is the answer when a value belongs to a pair rather than to either side of it. A fabric surcharge is usually different for each model, so it cannot live on the fabric and it cannot live on the model: it lives on the combination.
What to reuse
Troubleshooting
Repeat per related item is greyed out
It only applies to Item fields. On any other type there is nothing to repeat against.
The form suddenly has forty copies of one field
That is Repeat per related item doing its job, one copy per related item. If forty is too many, the relation is probably too broad; narrow what the item is related to.