What’s the difference between “sole source” and “single source?”

A single source procurement is a procurement in which only one source has been solicited usually due to a uniqueness of the service or technology requested. Other sources may be available using slightly different technology. Unless there are compelling reasons to support a single source, these should be avoided due to the perception of impropriety.

Conversely, a sole source procurement is where only one source possesses the unique and singularly available capability to meet the specification requirement and there are no other available sources or substitutions; e.g., replacement OEM parts, etc. This must be justified and backed up with the appropriate documentation.