A command can be either transparent or modal. A transparent command can be invoked when the user is being prompted for input. A modal command can be invoked only when AutoCAD is posting the command prompt and no other commands or programs are currently active. The commandFlags argument to the AcEdCommandStack::addCommand() function specifies whether the new command is modal (ACRX_CMD_MODAL) or transparent (ACRX_CMD_TRANSPARENT). The commandFlags argument also specifies other options for the command. See AcEdCommandStack in the ObjectARX Reference. Transparent commands can be nested only one level (that is, the main command is invoked, which invokes one transparent command).
If you create multiple commands that operate on a common set of global objects, consider whether you should make them modal so that they won't interfere with each other. If such collisions are not a problem, making new commands transparent results in greater flexibility of use.