Thank you for your suggestions. I will look into this

Hi,
Please go through our blog on how to set BOM in Odoo 16

https://www.cybrosys.com/blog/how-to-configure-bill-of-materials-in-odoo-16-erp

Hope it helps

Yeah, i am not really sure why but they explicitly disallow by-products on subcontracting ...

https://github.com/odoo/odoo/pull/75041

A quick test on 15.0 shows you can disable the constraint and it works.

​
class MrpBom(models.Model):    

_inherit = 'mrp.bom'        

	​@api.constrains('operation_ids', 'byproduct_ids', 'type')
	​def _check_subcontracting_no_operation(self):   	​
	​	​return True
	​

But for your case you can of course just create 2 boms, one for a 200m reel  consuming 1 reel and 0.2km of string and like wise for the 400m reel.

and order those together (perhaps even using a kit)