additional
Declares what this build can handle, not which release it is: the API reads this to decide which Fingerprint environment's key to serve. Saying nothing keeps the legacy key, which is what older builds get.
The aggregate endpoint carries it too: it is the only config request a normal launch makes, and its cached fingerprint block is what every later GetFingerprintConfiguration serves. Without the header here the whole process runs on a legacy key and never asks for a sealed one.