LAN Manager is a whole family of programs. Don't confused LanMan the family with the one implementation in NT -- LanMan is quite a bit older than WinNT.
LanMan is an opened-up version of 3Com's proprietary DOS-based server OS, 3+Share. I installed many 3+Share boxes in the late 1980s and early 1990s.
3+Share used NetBEUI but LanMan was protocol-neutral, which was rare and exceptional back then. E.g. AppleShare only ran over AppleTalk, Netware only ran over IPX/SPX, and Unix spoke unto Unix -- and nothing but Unix -- over TCP/IP. (Addons to run TCP/IP on other OSes existed but most of them cost money. Often more money than the OS itself, in the case of DOS. And many had proprietary APIs: so for example Quarterdeck DESQview/X used TCP/IP but it couldn't talk to the free TCP/IP stacks Microsoft and IBM eventually distributed. (Two different TCP/IP stacks, natch.)
LanMan ran on OS/2 1.x, on various proprietary Unixes, and on DEC VMS, which DEC marketed as part of its PATHWORKS suite: file/print serving via LanMan, plus Email, terminal emulation, X11 servers for DOS and Windows... all over the DecNet protocol.
NetWare was stable running vanilla file/print services (just don't load the AppleTalk module!), not so good with database services and so on.