In the documentation, it says that I can initialize with "complete set of field values, in .msg order".
What does this mean? And how do I implement it in Python and C++?
This simply means you can pass __init__ arguments in the order they appear in the .msg file. You would find the exact definition in the geometry_msgs Pose docs here. This is more typically seen as a python paradigm, and uses the * unpacking operator. Such as the following example:
def unpacking_func(*args):
print(f'I was passed {len(args)} items, they are: ')
for item in args:
print(item)
unpacking_func('first', 'second', 'third')
A specific example for the Pose you linked would look like this:
# NOTE: These are just empty values
position = geometry_msgs.Point()
quaternion = geometry_msgs.Quaternion()
pose = geometry_msgs.Pose(position, quaternion)
As a final point I'd note that you linked Diamondback docs, which has been EOL for a long time. This should all hold true for newer distros, but obviously there's no guarantee.