What does "one extra spell" in Herald's of Tzeentch rules mean?

[Mateusz]

Hi!

His rule says "If you roll a 9 or more for a Herald of Tzeentch's casting roll, it can attempt to cast one extra spell this hero phase (it must be a different spell)."

Does this mean that he can cast additional spells as long as he rolls 9+ or that if his roll is 9+ he can cast 2 instead of 1 this hero phase? 

I usually see it played the first way but I've seen a top player saying it's the other way.  Which one is true? Is there any official GW statement?

[Myzyrael]

As per my understanding, this should max trigger once per phase as the „one extra“ says you can get (only) one extra opposed to for each roll of 9.

But it isn’t exactly clear. I‘d say it triggers only once as if(condition) is checked once and forEach(condition) is checked every time this might happen.

my General rule of thumb: when in doubt go for the less positive way for you and the more positive way for your opponent.

[heywoah_twitch]

This rule is just very ambiguously written, and could be taken either way. I'm inclined to say that it's only once per phase since the Tzeentch book makes constant use of the word "additional" in many other rules, which would be the ideal word to use if they intended for a further 9+ to give you another spell after the first bonus spell. 

A FAQ would be helpful here to write it either:

"it can cast an additional spell" which would mean you can get multiples, or just add a parenthetical (max of one) if they intend for a limit.

[rokapoke]

It would be really easy to lump that into the Rules of 1 for generating extra attacks... it's the same principle. A spell generated by an "extra spell" cannot generate a further extra spell -- though this would be a rewording for GHB 2018, not rules as currently applied.