An Assembly must be created as a stock/inventory item.
There are two types of Assembly Pre-built and Ready to build:
Pre-Built Assemblies
These items are actually built in the assemblies area and the Stock reduced according to how many you actually build. When you’ve built how many you need (It also tells you how many you can build), you add that item to the Stock, knowing that the Stock has already been reduced. This assembly is treated as a single item in the Stock and also on the estimate and sales order.
Ready to Build Assemblies
These are assemblies you can create, but the Stock is not allocated until an order is placed for that item. When you select a quantity, if there is enough Stock, the item can be built and the Stock reduced accordingly. This assembly is treated as a single item in the Stock and also on the estimate and sales order.
To add the assembly to a Sale Order, select the assembly as you would select a normal stock item.