Custom Software Solutions are often refered to as NOTS or (Not off the Shelf).
A NOTS product is developed to meet specific requirements.
In the more general context, niche off-the-shelf refers to vendor-developed software that is for a specialized and narrow market segment, in comparison to the broad market for COTS products.
NOTS can be a full sytem as is often the case when existing solutions don't meet the client's requirements or small pieces of software written to integrate two separate systems.
For example, Salesforce and Facebook are examples of COTS, SAAS products.
However, if a client wants to publish all wins to their Facebook page automatically, the client would required NOTS integration to make that feature possible.