MS and DMS files contain dot molecular surface information in the format output by the programs ms (written by Michael Connolly) and dms (a reimplementation distributed with Midas). The format is described in the dms manual page and is referred to as "MS" below.
MS files can be opened in many ways, like other registered file types. The file type can be indicated with a suffix (.ms or .dms, part of the filename) or a prefix (ms: or dms:, not part of the filename).
The dot surface in a MS file is a molecular surface; in Chimera, however, the information is used to create a VRML model. Unlike an MSMS model, which is automatically tied to atoms in the corresponding molecule model (per-atom surfaces can change color along with the atoms, be recalculated when atomic radii are changed, etc.) the VRML model resulting from reading a MS file is an indivisible, unchanging object. The color is set once, upon opening. It is possible, however, to specify that a MS dot surface be associated with a particular molecule model. If that is done, the MS surface and molecule model cannot be moved or activated/deactivated for motion independently.
Surface-related Chimera menu items and the command surface pertain to MSMS surfaces only, not VRML models created from information in MS files.