You sure make things confusing.
No question about which way a frost proof unit must slope.
Not sure why the talk about all that other stuff.
I hate to add to the confusion but...
The frost proof design and the vacuum break design have two independent functions.
The frost proof design simply self drains when SHUT OFF and requires that no hose be attached.
My post is referring to FBHB without built in vacuum breakers, READ the last sentience of the post
The vacuum breaker design is to prevent back siphonage, [ syphonage] which of course can only happen when the hose bib is left OPEN and typically(but not necessarily) when there's a hose attached.
YES, That is the function of a vacuum breaker, Thank you for confirming that.
We are discussing frozen pipes
The point you are missing is when the HB is turned OFF, The diaphragm inside the VB
TRAPS water inside the HB as it opens to air break the hose
SO, what does that mean?
If the VB traps water inside the HB the HB can freeze