CAD tip # 2425:

• "'" (apostrophe) transparency prefix (invokes a command during execution of another command - e.g. '_HELP)
• "-" (dash) commandline prefix (invokes a non-dialog version of a command - e.g. _-LAYER)
• "_" (underline) non-localized command prefix (original english command names in a localized version of AutoCAD - e.g. _LINE)
• "." (dot) non-redefined command prefix (e.g. for commands undefined using _UNDEFINE, example: ._LINE or _.LINE)
• "+" (plus) dialog tab prefix (for selecting a particular dialog tab - e.g. '._+DSETTINGS), or a special command mode (e.g. +PUBLISH)

To list all commands which can be prefixed with "+", type "+" on the commandline and press the TAB key repeatedly (since version 2006). Similarly, to list all transparent commands, type "'" (apostrophe).