Solid Modeler was originally CorelCAD, and it was a bit shaky originally because the first ACIS kernel had problems, memory leakage and some other hiccups, but it ran alongside mainstream V5, which was 3D built on top of 4.1, with LISP command line and a BASIC interpreter, which was designer, deluxe and Pro with ACIS. Pro was an unstable horrorshow, frankly, but I began with designer and used it comfortably for a year or so. It didn't have 3D meshes or lofting, all you could do was prism and add them, or 3D faces with 3 or 4 vertices, and it only had one paperspace, so I used to use alternative viewports for each paperspace when needed. Solid Modeler was upgraded to 2.1 with ACIS 4, and became a lot better, I've still got it living on this PC. It's got associative history tree, shelling, lofting, sophisticated blending and edge treatments, scripting, LightWorks, output to .sat, .dxf, IGES (not very good) and .stl, almost the only accessible .stl output at the time, as well as .ccd, which was its own project format. It doesn't have a 2D environment, though, which is where TC 4.1 came in. V5 Pro crashes if you try to save .sat, and it claims to do IGES too, which quirkily disappeared in V6 and came back in later versions. They stopped selling Solid Modeler during V6's run, and if you try to install SM it tells you that it's no longer supported - but if you install it, then upgrade to 2.1, it'll register online and run. Then they did a better job of ACIS integration in V7 with shelling and blending and it became a proper hybrid being able to switch between solids and surfaces. Solid Modeler is still pretty impressive for two decades and more back. The first version inflicted reputational damage that it didn't recover from, unjustly, IMO. When SolidWorks was first developed, they tried ACIS and Parasolid kernels, and very quickly settled on Parasolid because of ACIS' problems. ACIS has been well developed since then, even AutoCAD has it, although Autodesk bought a permanent license at V7 and then went their own way, but if you want to import TC solids into AC, save them as .sat V7 and they'll pass.