Both durum and semolina flours are typically, but not always milled from durum wheat. Their main application is in the production of pasta. Durum flour is indeed of a finer consistency than semolina flour which is milled differently than durum flour and has a coarser texture (larger particle size). The gluten from durum wheat is somewhat different from the gluten of regular hard red spring and winter wheat varieties in that it is much more elastic/tight. If you buy pasta made using a durum wheat flour and one made from a hard wheat flour you will readily see the difference as the pasta made from the hard wheat flour has a softer, slightly gummy texture, while that made from the durum wheat has a firm texture. By the way, you can readily tell the two apart at the supermarket without even needing to read the label, the durum wheat pasta will be a light creamy, almost yellow color, while that made from a hard red wheat flour will have a dark, almost dirty/dingy color. It typically sells for a lower price too.
It looks like you have access to General Mills Flours. Here are some typical protein numbers that might be of interest to you for the GM flours.
Rex Royal: 12.4%; Washburn's: 12.6%; Full Strength: 12.6%; Superlative: 12.6%; Hi Power: 13%; Remarkable: 13.6%; All Trumps: 14.2%; WINTER WHEAT FLOURS: King Wheat: 11.1%; Doughbuilder: 11.1%;GM-44: 11.1%; Pollyanna (Untreated) 10.5%; Harvest King: 12%; Ben Hur (Unbleached) 12%; Semolina #1 (Enriched): 12.2%; Extra Fancy Durum: 12.2%
Tom Lehmann/The Dough Doctor