Under what conditions what you consider single-sourcing an time in the leveraging category of the portfolio matrix?

Under what conditions what you consider single-sourcing an time in the leveraging category of the portfolio matrix?
The single sourcing strategy means that a company is getting the supply from a single source, for a predetermined time period. The level of asset specificity of the supply may vary from medium to high, since the supplies are directly related to the buyer’s core competitors.