Okay, let me lay it out for you;

The RP will begin pre-formation of the new formation - that'll be the one our characters come together to create later on. At the beginning we are nearing the end of an Imperial crusade against the Greenskins in an overrun system of Imperial space, the regiments we are currently part of will be folded together to create a mixed/composite regiment of varying peoples, fighting types and so forth.

That's pretty much all you need to know.