Sole Source
Buying without competition because only one vendor can genuinely meet the need. Allowed in most jurisdictions, and required to be justified in writing.
Sole source is the exception to competition. It applies where only one supplier can actually provide what is needed: a proprietary component, a system that must integrate with what is already installed, a warranty repair only the manufacturer may perform.
The justification is the whole of it, and it has to survive being read later. The test is whether one vendor can meet the requirement, not whether one vendor is preferred, more convenient or already familiar. Inconvenience of competing is not a sole-source basis, and neither is a specification written narrowly enough to leave one candidate.
In fleet the legitimate cases cluster in a few places: parts and service tied to an existing platform, software that must connect to an installed system, and equipment where a single manufacturer holds the capability. The illegitimate ones usually start as a preference and get reverse-engineered into a justification, which is exactly the pattern an auditor is trained to spot.