Raven Guard: my head canon is the chapter needs to venture out from Deliverance to find Corax, due to further issues with geneseed that has limited their resources.
As a result Shrike has assembled a special company under his direct command comprising of several specialist units from the 10 companies of the chapter. Deployment varies depending on the situation, could just be a small taskforce for covert ops, or combination of units for more complex missions. They are not always comprised of the elite of the Raven Guard, but are most certainly the optimum combination for the objectives at hand.