Also not mentioned, they’re not unique across time: six base-36 characters is only 2 billion possibilities, wouldn’t surprise me if the largest GDS would blow through the entire space within a year. <https://support.travelport.com/webhelp/smartpointcloud/Conte...> suggests they get purged after a week, and recycled.

I wonder what fraction of the space is occupied at any given time.

There used to be a story going around, possibly apocryphal, that the process to reset them used to be entirely manual and generally unknown to the rest of the organization. There was a big problem the week the person who had that task retired. Someone around here was probably there if it really happened and can add some color.