Field help (PTOCOD) 

The assignment rule is used to create a link between a requirement (sales order, material requirement, etc.) and a resource (work order, purchase order, etc.).
This function, which can be accessed via a tunnel or a right click, makes it possible to specify the link to be created: if box "Direct order (1 to 1) mode" is ticked, the system will authorize the assignment of a sales order to a purchase order, for instance, but not the assignment of a sales order to several purchase orders (it must be 1 to 1).
This rule is applied upon requirement evolution (order update or creation).