How does Shopify group items into fulfillment groups?

[Lanpham]

Hi there,

I'm learning about the Order Routing Location Rule. There's an input object called FulfillmentGroups - A list of groups representing the items to be fulfilled (A fulfillmentGroup is a group of one or more items to be fulfilled together).

I have 1 question about how Shopify combines items into fulfillment groups?

Thanks!