Sole sourcing
When only one supplier can provide a part, because of a patent, a proprietary design, a certification or a specification that names them.
Also called: Sole source, Captive supply
A part is sole-sourced when there is no second supplier to call. Sometimes that is real: a patented sensor, a qualified aerospace casting that would take 18 months to requalify elsewhere, a supplier who owns the drawing.
Often it is self-inflicted. The engineer wrote a brand and reference on the drawing (“SKF 6205-2RS or equivalent”, with “or equivalent” lost somewhere along the way). Or the tolerances were copied from a previous project and are tighter than any other supplier will accept without a price premium.
Telling the two apart
The useful question is: what exactly makes this supplier the only one? If the answer is a property of the part (a material, a process, a certification), it can be checked against what other suppliers can do. If the answer is a line on a drawing, it can often be changed.
A bearing specified by manufacturer reference is a typical example. The same ISO dimensions, sealing and clearance class are available from four or five makers, at prices that can differ by 30%. Rewriting the spec as dimensions and performance, instead of a part number, turns a sole-sourced part into a single-sourced one, and then into a competitive one.
When it is fine
Some sole sources are worth keeping: the supplier is the best at it, the volumes are small and the part is not critical to cost. The point is to know which ones you chose and which ones you inherited.