An ancillary product is a product which is purchased or licensed on a restricted-use basis, in conjunction with the principal manufacturer's product being acquired (e.g., may be used only in combination with or by educational institutions for research use).  For instance, a third party software which is licensed to licensor and furnished to licensee on a limited-use basis subject to the license restriction that it may only be used in conjunction with the licensed software. 

Source: www. PnpPsgGlossary.doc