A shuttle service is the same as a multi project wafer.
On 30th June 2020, Tim Ansell announced the collaboration between Google and Skywater to provide a free shuttle service in his FOSSI dialup talk.
These slides taken from his talk show the overview:
Normally in an MPW you would have the whole die to yourself. In the Google/Skywater Shuttle all the designs will be within the user project area of the Caravel harness.
Caravel has its own RISCV processor and a neat datasheet.
The idea here is to make it easier to bring up and debug your designs at the cost of some area.
Read this post to find out what you could fit in the user space.
The Google shuttle will return chips in type of packaging called Wafer Level Chip Scale Packaging.
Efabless have put up a page that has more details on the requirements for entry to the shuttle.
I was like holy cow I can make my own CPU? I mean come on this is kind of fantastic making your own CPU. I gotta jump on that train!