A W9M is the CASS aircraft type for Endeavor Air CRJ-900s. Each of Delta's subfleets can be referred to by three different abbreviations: the Indicator type (most basic), the aircraft type (common), and the CASS type (mostly for internal use). Endeavor Air CRJ-900s are CR9, CM9, and W9M respectively. I believe the common use of the letter W in the CASS types indicates Comfort+ seats because they were added when Comfort+ was being rolled out, but that specific factoid is just a rumor I've heard. (Edit: Now that I re-remembered that CASS=computer-assisted seat selection, I think this 'rumor' is true because the new seat maps with Comfort+ as its own fare are different than the old seat maps. Comfort+ is now displayed in its own cabin instead of combined with Main Cabin, so the W letter in the CASS aircraft types means that the aircraft's seat map has a segregated Comfort+ cabin.)

Some others:

• CR9, RJ9, WR9 (SkyWest CRJ-900)
• 319, 31J, W1J (132-seat A319)
• 320, 3KR, W2D (160-seat A320)
• 717, 717, W17
• 73H, 73H, W3H
• 739, 739, W39
• 757, 75D, W5D (199-seat 757-200)

Aircraft types with only one subfleet (like the 717) have the same Ind and AC types. Aircraft with more than one subfleet (like the 757) have more than one AC types (like 75D, 75S, 75G).

Edit: I don't think the CASS types are meant to be used on delta.com. I think the Ind types are intended for use because they're more recognizable and have been used most the time.