Evaluate the collective function of references cited together in a given text segment.
Classify their combined role into one of the following categories: 'method', 'background', or 'result'.

Text:The ABF procedure was preferred for (1) patients with long segment occlusions (combined common iliac and external iliac occlusions) that were heavily calcified and with associated infrainguinal disease, (2) patients with technically failed percutaneous intervention, (3) patients with recurrent stent thrombosis after failed percutaneous secondary intervention for iliac artery occlusion, and (4) patients with a hypoplastic aorta and small-diameter arteries distal to the iliac artery occlusion.
Category: