They are additions, but are, at present, by opponent consent only. the
IA books are a supplement to
40k.
You can use them just fine in any game, as long as the other guy/
TO allows you to.
They don't have the official-ness of an actual codex, and most tournaments will only allow
40k codex units, though many of the
FW models are just fine to use if they're in the codex.
One of the reasons that they aren't generally allowed in tournaments is that the points values and power levels of the
FW units are often out of whack, either too low or too high, and many
FLGS or even
GW stores don't have copies of the
IA books on hand, which would be needed to make sure the
FW units were being used properly and to check points in the lists.
I've found with my local tournament that i can roll up with a
FW nurgle Daemon Prince (for example) and it's no problem. People love seeing the models on the table. Bringing Blight Drones (brokenly
OP nurgle
IA unit) would get them thrown at me.
FW models are often fine in any game (and damn pretty)
IA units are generally a no-go in a competitive setting.
*edit for dyslexia

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