What is the `search_id` in Planet's Quad Search Results API call?

Planet's API documentation shows what appears to be a general search function:

https://developers.planet.com/docs/basemaps/reference/#operation/getQuadSearchLinks

(specifically https://api.planet.com/basemaps/v1/mosaics/{mosaic_id}/quads/search/{search_id})

But it does not document how to construct the search_id.

What does this API call do and how does one get the search_id?