As you get higher in levels and get Fire III and Blizzard III, you'll notice that the wording is a bit different compared to the lower Fire and Ice spells.

"Grants a full stack" - while the wording is a bit strange, I'll tell you exactly what it does.

Fire 3 - Will always give you max stacks of Astral Fire (even if you use it during Umbral Ice it will give you Max Astral Fire without using Transpose first)
Blizzard 3 - Will always give you max stacks of Umbral Ice (even if you use it during Astral Fire it will give you Max Umbral Ice without using Transpose first)

I wont give away too much, but basically that information will be key to becoming a successful Black Mage.