SIStoreAction is a low-level command implemented in C++. "StoreAction" is a front-end command implemented in VBScript, in action.vbs actually called "StoreActionProc", which calls SIStoreAction after the command arguments code has done its magic (perhaps parsing the selection list, for example), and then I think it adds it to the mixer. You can look up the source in your softimage install folder
We have quite a few commands that are implemented in vbs or js, which are named "Proc". Get familiar with these files.
