Create your first rule
Create a rule to decide which supplier receives an order line. For example, send all products of one brand to the supplier that stocks them. You need at least one supplier first.
In the sidebar, under Automations, click Rules, then New rule.
Choose a field, an operator and a value: for example Brand, Is and the brand name.
Choose the supplier for the matching order lines, and save.
Result
The rule is added at the bottom of the Rules list, named Rule. From then on, order lines that match it go to the supplier you chose, unless a rule higher in the list matches first.
Put your rules in order
Dropday checks your rules from top to bottom, and the first rule that matches decides the supplier of each order line. Put specific rules above general ones by dragging a rule by its grip handle on the Rules list; the new order is saved right away.
Order lines that match no rule
An order line that matches no rule shows No rule and isn't sent to any supplier. Rules run when an order comes in, so a new or changed rule doesn't apply to order lines that already arrived. Check a new or changed rule with a new test order instead, as described in Test your setup.
If this happened to a real order, An order line was not sent to a supplier explains your options.
Refine a rule
Open a rule from the Rules list to rename it, add conditions or set a purchase-price threshold. How rules decide the supplier lists all fields and operators.
Next, test your setup with a test order.