1 - Trying to find a DoD accepted DEFINITION of Sole Source and Single Source. Not a copy of the FAR subpart 6.3 Other than full and open competition, but a true definition of each that is accepted DoD wide to include in a glossary of terms.
2 - Is sole source considered a good or service with only a single SUPPLIER/VENDOR? Or is it a good or serivce with only a single MANUFACTURER that may or may not have several distributors?
The DOD 5000 glossary defines "sole source" as "A contract for the purchase of supplies or services that is entered into or proposed to be entered into by an agency after soliciting and negotiating with only one source." It does not define "single source". Specific Agencies or Organizations may define single source, however you are unlikely to find a DoD definition for single source because it is really not relevant to supporting a sole source procurement. Procurements may be made on a competitive basis, a sole source basis or with limited competition.
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Under a sole source procurement, an agency determines that a proposed contractor is the "only source" (vice single source) capable of meeting its technical needs and provides a reasonable basis for its determination. The basis may be urgency of need, duplication of cost, uniqueness, specific technical features etc. These are the factors which drive the procurement to a particular source, which is rarely the only source, but rather the best source for the instant procurement. Rarely will you find a sole source determination based on an argument of a single source. Rather it is the impact of other factors which supports the rationale of going to "only one source" which is the terminology used in the FAR/DFARS and most procurement related sources of information.
Rather than focus on trying to find a definition of single source, it is best to understand that the use of the adjective "sole" relative to source is based on the procurement being not divided or shared with others rather than there truly being only one entity capable of supply the goods or service. The questions asked cannot be answered broadly because each procurement must stand on its own merits.