memo means NON-inventory item, i.e. not affecting on-hand inventory
(while affecting financial/accounting data)
e.g: when you want to credit a customer, you create Sales Order Line with
item like: Credit invoice#1234
item which is not defined in item master automatically defaults to Memo
Note: if item do exists in item master, you can still override it manually, by
entering 'M' in Sales Order Type field