I have read a number of threads on this forum, and read the books carefully. There appears to be confusion about what "Immunity to Normal Weapons" means.

I would quite like to have feedback on what it means. My understanding is as follows:

The following assumes a spirit with ITnW 12 (i.e. a force 6 spirit), and Body 6)

• vs a weak punch: A punch does 11P damage. In this case the spirit takes no damage.
• vs a strong punch: A punch vs 13P damage. In this case the spirit soaks with 13P damage with 18 die (ITnW 12 + Body 6)
• vs a mundane weapon: An axe does 11P damage. In this case the spirit takes no damage
• vs a weak single round: A pistol does 11P damage. In this case the spirit takes no damage
• vs a strong single round: A pistol does 13P damage. In this case the spirit soaks with 13P damage with 18 die (ITnW 12 + Body 6) takes no damage
• vs a weakish single round with AP. A pistol does 11P damage with AP -2. In this case the spirit soaks 11P damage with 16 die
• vs a burst. A short narrow burst does 11P after resolving the nett successes (but before adding +3 burst modifier). In this case the spirit takes no damage.
• vs a taser. The taser does 6S(electrical). The spirits "hardened" armour of 12 is halved to 6, but the spirit still takes no damage
• vs a stronger taser. The taser does 7S(electrical). The spirits "hardened" armour of 12 is halved to 6, The spirit has to soak 7 damage with half armour (6) and body 6
• vs sonic. The sound pistol does 5P damage. I have no clue what happens to the spirit...
• vs magic. The whole immune to normal weapons doesn't apply, and you soak damage with just body

To summarise the above:
vs mundane attacks you get 2*force hardened armour
vs Armour piercing attacks, the armour piercing works
vs elemental attacks you get force hardened armour
vs magic attacks you get no armour.

Does this agree with your view of the rules?

This is how I see it. There is one modification that is strongly argued, in favour by some, against by others: for purposes of a weapon's AP, it can be interpreted that the AP applies once for the hardened armor of ItNW, and again for the regular armor (if any) worn by the spirit or the vessel it possesses (if of the appropriate tradition). There is a quote somewhere on DS from a developer (or two?) who argued in favour, but that is not RAW by any means.

Proponents for this interpretation tend to fall into the camp that believes possession spirits are too powerful, and anything that weakens them is a 'good thing'.