Chullin 93a: Cheilev that is covered by the meat is permitted. Rashi: Forbidden fat that is on the flanks under the kidneys and visible on the high part of the flanks, and then disappears under the meat, red and spreading under the meat throughout the flanks. As soon as the meat covers it, it is permitted. And when that meat ends below [at the animal’s belly], a thick, white membrane protrudes… under that thick membrane there is cheilev, and some hold that that cheilev is forbidden, because that membrane is not considered “a covering of meat” since it is very thin. But in Germany they permit it. And to me as well, it seems that it is a complete covering.
חולין צג ע”א: אמר רבי אבא אמר רב יהודה אמר שמואל: חלב שהבשר חופה אותו מותר. פירש”י חלב שהבשר חופה אותו ־ חלב שעל הכסלים שתחת הכליות ונראה בגובה הכסלים ואח״כ נבלע תחת הבשר אדום דק ומתפשט תחת אותו בשר בכל הכסלים ומשהבשר חופהו מותר וכשכלה למטה אותו בשר יוצא ממנו קרום עב ולבן, לבד קרום דק וקלוש המתפשט בכל הכסלים שאסור משום חלב כדלקמן, ותחת אותו קרום עב יש חלב ויש שנוהגים איסור באותו חלב לפי שאותו קרום אינו חשוב חפוי בשר שדק הוא אבל בארץ אשכנז נוהגין בו היתר וגם בעיני נראה דחפוי גמור הוא.
Rabbi Yisroel Belsky was once asked to inspect a traditionally reliable kosher restaurant, after someone suggested that they were serving cuts of lamb meat with the cheilev still attached. This establishment would buy the freshest meat and perform their own nikkur, and they employed a mashgiach with many years of experience. “The owner of the restaurant, the butcher, and the mashgiach each inspected the lambs they showed me,” Rav Belsky related, “and were at a loss to explain why anyone would suggest there was any cheilev. Now, all lambs have cheilev attached to the navel, and even though that piece of meat is very tasty, there is no way to clean off the cheilev, and it must simply be cut off and discarded. Unfortunately for this restaurant, they had never identified this piece of cheilev for they mistakenly looked at the meat upside-down, and thought the cheilev was simply a piece of permitted fat. The allegation was indeed true.”
Source: Halachic Responsa From the Desk of Harav Yisroel Belsky, p. 191