It’s probably simple but I don’t get the write syntax to restrict the domain of a party Many2One list to a category created in party_category and linked in party_category_rel table.
party = fields.Many2One('party.party',"Supplier",domain=['id','child_of', ... ???
Without showing more code (model, fields that condition), I only can answer what it’s normally done, the domain you should be doing something similar to: ('categories.parent', 'child_of', [category.id]),