According to the documentation for Google Speech Custom Classes, the URL takes a locations parameter, as documented here:
The projects parameter I assume is just the name of the project on my Google cloud account, but what is locations and where can I get it?
The principle is to have the capacity to locate your data in case of specific local regulation.
However, global is used in all the example. I tried to put anything in the location, and anything is a valid location... Not sure that this parameter is active in Preview.
Use global for now, it's the safer and the most compliant with the future. And wait the API in GA to get the official list and an implemented feature.