Here is my interpretation assuming this is accurate.
Group A - almost no chance one of these players falls to 18. Would consider a trade up if one of these players falls into the teens (with the most likely being Star).
Group B - will select from this group if one of them falls to 18 but would not trade up to get. Cowboys feel unlikely that one of the players will fall to 18 unless one of the OG's slide.
Group C - most likely to get one of these players at 18. The odds are very good that S Williams will be available so he may be the mostly likely selection at this point. Question: would Dallas take the CB Rhodes if all the other Group A, B, and C players are gone?
Group D - I think Vacarro is the last player with a first round grade. The Cowboys only take Eifert or Patterson if all other first round graded players are gone and they can't trade down from 18.