It can also be invoked from within a program, but at the sacrifice of the interactive capabilities that are its true strength. SM has some capability to handle image data. A few standard types of binary files have been defined which should, we hope, cover the majority of cases. SM is not for sale to groups who want to use it for weapons research projects for the military agencies of any country. When he gets his act together, there'll be an archive available on the home page.
Data can be numeric or alphabetic.
Quite a few macros are included in the distribution, and they are helpful when learning to write macros. Yes, if used as subroutines in your own program. Interactivity provides a lot of flexibility. IDL allows you to manipulate your data interactively, and then plot the results, which is very nice, but the cost is higher.