Joining the packet sources

[xoft]

Those flags are just a bitfield, similar to how some block metadata work in MC. Bit 0 says if the player thinks they are in Creative mode, bit 1 says if they think they are flying, bit 2 says if they think they are allowed to fly, etc.
To construct such a value, you OR the bit values together:

Byte ValueToSend = 1 | 2 | 4;  // Creative, flying, can fly

To query such value, you AND it with the respective bitmasks:

if ((ValueReceived & 1) != 0)
{
  // Creative
}
if ((ValueReceived & 2) != 0)
{
  // Flying
}

Note that the vanilla server ignores some of these, we should as well.