What is the hood on AF, 8 cal, jackets for? I don't think this can be used instead of a balaclava or am I wrong?

IIRC, arc flash hood is defined (in NFPA70E) as a beekeeper-type hood. Now, if you know beekeepers who use hoodies...

Balaclava was added due to two concerns:
1. With just a face shield the hot gases can go "around the corner" behind the face shield.
2. You are not always going to be facing the direction of the arcing fault and may get hit at the back of the head.

That being said, I can easily see where a "parka" style hood with a button up mouth/nose protection would be identical to a balaclava.

Although 70E gives specific clothing recommendations associated with the tables, if you are using calculated incident energy values, then as explained in the annex, the PPE table goes away too. Using the table as a recommendation based on an engineering study of arc flash hazards is "backwards" and not what the table is intended for. In this case you go by the manufacturer recommendation and if they recommend a clothing system including face shield, jacket, hood, etc. that is specified for a specific cal/cm^2 rating then 70E allows this. We are seeing this now with a couple manufacturers that are manufacturing clothing with significantly lower weight cloth (oz/yard) that meets the required incident energy ratings.

Some of the flash jackets have a hood which covers the area a balaclava is designed to cover and would meet the requirement for NFPA 70E. Most "hoodies" do not cover the chin and would not. Would it be pretty good? Yes. But not the entire area in the description from NFPA 70E.

I think it depends on the area it covers, not what you call it. A hoodie might not meet this requirement. Some I have seen have a velcro flap to cover the chin and neck well.

I think MOST of the side of the head and some of the neck would be covered well with a hoodie and a faceshield with chincup. New things are coming out of the EU that make hoods much more comfortable with much less fabric. CATU has a new shield that folds up into the hardhat and has fabric on chin and on side. Salisbury new 40 cal hood design opens to allow seeing and breathing and from what I have seen doesn't fog at all. Pretty amazing stuff coming online. Great time to be in the industry. Love testing the new designs.