When you click Add New Item on a CRM Order, the dropdown menu in the modal that appears will display all the available SKUs for that order.
The only SKUs available are the SKUs whose sales class matches the order's type, which is based on the type of the CRM Account the order is for.
Here's an example. An order is created for CRM Account X. Account X's type is "distributor", which makes the order type a "wholesale" order. There is a SKU called "GummyBar-1g" whose sales class is "whole sale". Since the SKU's sales class is the same as the order's type, it will be available for selection when you click Add New Item on the order.
