Rather than thinking of AutoCAD as “running in 2D or 3D” think of a giant 3D volume (the AutoCAD world space). Lines, arcs, text, dimension line etc. can be placed anywhere and at any orientation in that 3 dimensional space. Users who only need to work in 2D can ignore the 3D aspect of the world they are working in. For the 2D users, when they add a line from 1.2, 3.1 to 4.3, 5.5 they are actually adding a line from 1.2, 3.1, 0.0 to 4.3, 5.5, 0.0. The z coordinates for everything they do is automatically assigned 0.0. AutoCAD will typically add a 0.0 z coordinate if only x,y coordinates are specified.
AutoCAD uses the term “workspace” which I think is a bit misleading. A workspace is simply a label for a set of AutoCAD functions that are displayed in the user interface (primarily the ribbon at the top of the screen). The “Drafting & Annotation” workspace displays a wide variety of features that are important to 2D users. You will not see 3D features such as creating boxes, cylinders and other 3D primitives in the ribbons. Change the workspace to “3D Modeling” and you will now see features for creating a Box, extruding a 2D shape into 3D, as well as 3D Move and 3D rotate. No matter what the active workspace however, you can work in 2D or 3D. For example, when the “Drafting & Annotation” workspace is active you could give the line command followed by x,y,z coordinates for the line’s endpoints thus creating a line in 3D while working in the supposedly 2D drafting world.
It is not clear to me if you want to work in 3D or stay in 2D and are just concerned that you may be inadvertently creating a problem with the 3D potential being there.
lee.minardi