From: Berlin, Germany
I think it is the checkable option "Carrier planes" on the "Additional Units" tab. <rclick> will produce a text about what this checkbox does, in the usual vague language, which gives the impression the true content can be read up elsewhere.. probably one of the Players Manuals.
For experienced WIF players, it should be noted that this optional rule automatically incorporates the
optional rule Carrier Planes CV Only from RAW.
I am not a native speaker, but ... . It could mean that by checking the option you will now play with CVP counters and that automatically SiF+CVPiF are active. That would mean you automatically play with double stacking, as this is an automatic option if playing with CVPiF. That is what I see ingame at least with this option checked.
CVPiF also adds the juicy 44-47 planes should you go for advance building. Painfull without air specs though.. at least for JP, which profits the most from it. I am not sure I would prefer a game with only SiF CVP (w/o option CVPiF 46), but then we got CVPiF as an extra when we purchased the Deluxe Box at our store and never played without.
“WiF is like sex: sure, it may give some practical results, but that's not why we do it.”
- Richard P. Feynman, 'WiF, Sex, and the Dual Slit Experiment'.