Can be used to refer to a relationship between two software applications. Example: A client company has an ERP application installed. Examples of a one-offs would include applications which interface with the primary application (i.e., the ERP software) such as:
1) CRM (Customer Relationship Management) application
2) Specialized Warehousing software
3) Barcode-reading software
4) Quote-generating software
5) etc.
This terminology is used by software application companies to designate sales opportunities within their client base or other potential customers who use the aforementioned primary software application.