Here's is my take on this...
CG9. CONCEALMENT: All Infantry (and their possessed Weapon) setting up in Concealment Terrain may always be set up concealed if out of enemy LOS. Otherwise, A12.12 applies normally. Other eligible units may be concealed prior to the opponent viewing the setup. “?” purchased in the RePh are intended mainly for use as dummies.
CG15. CONCEALMENT: All Infantry (and their possessed Weapon) setting up in Concealment Terrain may do so concealed, regardless of LOS. Such “?” need not be purchased; i.e., they are placed freely. Otherwise, A12.12 and A12.34 apply. The side setting up first may conceal all eligible units before the other side views its setup; likewise, the side setting up second may conceal all eligible units before his opponent views that setup. “?” purchased in the RePh (11.6121) are intended mainly for use as dummies.
The red text is what differs between Manila and Red Barricades. The blue text in CG15 is, IMO, really not needed, just an extra clarification. The "general" part about "Otherwise, A12.12 apply." is present in both SSR.
The green text says - again IMO - the same thing....just in different words.
So the main difference is the red text. Lets say I set up first, and my opponent has Isolated units left onboard from the previous CG scenario. If I set them up in Concealment Terrain in LOS of those Isolated units, I don't get Concealment for free - I would need to use purchased "?" counters. In Red Barricades I could set them up Concealed "for free".
Besides this, any unit I set up outside of the Isolated units' LOS I can place "?" on - per the green text. Then my opponent does his setup, with the same restrictions.
So, technically, I believe that "setting up in Concealment Terrain" in the first sentence of CG9 might be somewhat superfluous, since getting a free "?" if setting up out of LOS applies to all units.
Of course, I could be missing something...
