Compare:
1200 If the Will Flag is set to 1, the Will Topic is the next field ...
1203 If the Will Flag is set to 1 the Will Message is the next field ...
with:
1212 If the User Name Flag is set to 1, this is the next field in the payload.
1216 If the Password Flag is set to 1, this is the next field in the payload ...
I prefer the first style - so "If the User Name Flag is set to 1", the User Name is the next field in the payload".