I can't speak authoritatively on this, because I haven't looked at restore CDs from the era yet, but the AWGS machines were mostly marketed to particular potential with add-on software you could get, both from Apple and from third parties, and less to (as in the later years in the G3/G4/OSX-Server era when Apple definitely bundled AppleShare IP and later OS X Server) the built-in capabilities, of, for example, AppleShare/IP.
With that in mind the impression I've always gotten from 6150/8150/9150 materials is that the three were segmented based on intended use cases as well as workgroup/user base sizes. i.e. the 6150 might "prototypically" have served a single team of 20 or fewer people with a couple hundred megs of shared data and one or two printers, or run, alongside other machines or bigger ones, a single application or task (groupware like email/firstclass or a calendar application, or host a ClarisWorks database as part of a line of business apps.)
For example, see:
1993 Magazine ad:
http://www.vectronicscollections.org/gallery/ads/macintosh/0139.php
1993 Internet Server ad (featuring an AWGS60)
http://www.vectronicscollections.org/gallery/ads/macintosh/0158.php
1993 AWGS95
http://www.vectronicscollections.org/gallery/ads/macintosh/0165.php (I know this is a different scenario b/w A/UX but womewhere there's an AWGS 95 configuration document that talks about buying it with/without A/UX, and with/without licensing to different service components.)
1993 68k-based AWGS brochure
http://www.vectronicscollections.org/gallery/ads/macintosh/0230.php - talks about different models, gives examples of why you might want a "server" in the first place. Also, gives a list of example applications you might run on a server. There's a fair number of different appliancey things that you might expect to run together on the same box and/or in virtual machines today that probably wouldn't have in the '90s for speed and reliability reasons.
1994 68k/PPC-based AWGS brochure
http://www.vectronicscollections.org/gallery/ads/macintosh/0232.php - similar, updated for PPC. Honorable mention, here, to PowerShare, which existed for all of like three years as a high end powerTalk coordination and relay service for messaging and for powertalk file and coordination/directory stuff. A 9150 or 8150 would've made a pretty swole enterprise-wide PowerTalk directory server while a 6150 might have been a good departmental/team filebox or suited to a single task like bridging powertalk and internet-based email systems.
1994 AWGS magazine ad (1)
http://www.vectronicscollections.org/gallery/ads/macintosh/0140.php
1994 AWGS magazine ad (2)
http://www.vectronicscollections.org/gallery/ads/macintosh/0141.php - pretty generic, describes headaches of the time to be solved by having a server.
1997 AWGS article
http://www.vectronicscollections.org/gallery/ads/macintosh/0275.php - talks about why you might need/want a server. 7250/7350/9650.
The 1997 brochure is really explicit about there being different software bundles for different ways you might have used a server, so you might, in 1997, have bought a 7250 to run a web site, another 7250 to run email, and a 9650 to stuff hard disks in to do local file service, and a 7350 to do a particular workflow task,
for example.
Annoyingly, to the point of the original question, the 1994 68k/PPC AWGS brochure, which is the one that mentions the 6150, doesn't actually list specific server bundles, but Apple went hard on Mac servers running both their own and third party utilities for most of the '90s so it basically tracks that there was an appleshare 4 + powershare version, a version that was pretty much blank (just like a stock pmac6100/ would've been) and a version that, say, included some internet-focused software. (Especially since AppleShare IP 5 in like 1997 was the first version of AppleShare to bundle Internet-based services like web and email,
For as bad as Apple's model proliferation problem was in the '90s, I've long felt they would've been well served, perhaps using clone builder techniques or even literally clone builder motherboards, by building some systems a little more tunes, physically, to service needs. A box with room for a couple more hard disks without being "the 9650/233" or, IDK, something with more serial ports (even as an add-in card) for peripheral/printer sharing and/or faster localtalk routing/bridging, for example, would be a neat thing to have seen. (I would build this as a variation of the 4400 or something along the lines of the PowerComputing PowerBase or UMAX C600, but in a bigger case, perhaps even do an IDE boot disk and room for Floppy, CD-ROM, [5.25-inch removable device] and then three 3.5-inch disk bays, bundle a software raid tool that can build a raid 5 stripe.
Given the not-particularly-wide proliferation of AWGSes, I imagine that it would've made negative sense to build that as a product. When Apple did build (often, pretty good) purpose-built hardware (ANS, XServe) I feel like they aimed it at the wrong market. The XServe should ahve been a 4u or 5u server with 8 or so 3.5-inch hard disk bays on the front, that you can run as a minitower in a closet, behind a photocopier or under a desk, and the ANS should have been an AppleShare IP box for only a little more than the 9650 cost with a focus on some more configurability and the disk bays in particular. (There was an ANS300 that was also rack-mountable that got as far as a prototype state, overall specs-wise I think it was comparable to a 7300 or 8500, but I forgot what its storage arrangements were.)
.... Anyway.
