The special rules for IG fortifications state that they can have them, were to place them, but not what type or how many they can have. There is even a reference on pg. 42 of the new 2012 army list compendium concerning paying for fortifications, but no points listing:

"Once set up, fortifications may be used by any unit, not just the units they
were purchased for. They may be captured and used by enemy units too".

So, can anyone clarify on exactly how these work for IG?

Of the Compendium armies, only Baran, Death Korps, and Cadian Armies may use Fortifications. Costs are in the respective army lists. Effects are in the Death Korps army list on p48 (p50 of the PDF), under "Death Korps Terrain Table". Death Korps are listed as being able to take Razor Wire, but this is an error; they may only take 80cm of Tenches and 4 Bunkers per set of fortifications.

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SG

Ghost's Paint Blog, where everything goes that isn't something else.

As Spectrar notes, the fortifications allowed are in the respective army lists.

Incidentally, there is a Marine list or two that has fortifications as well.

That's right. The Imperial Fists have fortifications as well.

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SG

Ghost's Paint Blog, where everything goes that isn't something else.