Beveling refers to the shading of graphic lines (typically used as borders) to give them a three dimensional (or "beveled") appearance. This can be classified as nothing more than visual "gingerbread", but it can also make certain applications more pleasing to the eye (and thus perhaps more enjoyable to work with). The menu selection brings up a dialog box, which has beveling options of "disable", "automatic", or "program control". Predictably, the first one disables the effect while the second option causes it to be turned on automatically. The last option allows applications the option of turning it on or off at their own discretion.
The SET.LIT command line SET TERM BEVEL <option> can be used to change the beveling setting within an CMD or DO file. Available <option> choices are OFF, AUTO, PROGRAM-ON, and PROGRAM-OFF.