Accessing local fields

[corvus-migratorius]

The tool issues a warning against accessing local fields by their name alone (saying it's "unstable") and instructs us to use the format illustrated by 6063181a59590578909db920--localField (let me call it the "full" format, for the lack of a better term) instead.

Could anyone please explain, where does the 6063181a59590578909db920 part come from?

Thanks a lot!