In the beginning there was a sort of hierarchy with the addition of modifier keys, it goes cmd, shift, option, control.
Basic commands use the command key with a letter. Adding the shift key usually reverses the command, like cmd-Z in Finder is Undo, cmd-shift-Z is Redo.
The addition of the option key is supposed to add an alternative or extension to the basic command like when you see commands whose text contains an ellipsis (which means that the command will bring up a dialog for more information before executing). Adding the option key can remove the ellipse and just execute the command with the last parameters that were set. In the Finder, cmd-W closes a window, cmd-option-W closes all windows.
Rarely would you find a command that uses all four modifier keys, it's just hard to press them all at the same time.
Of course over the years and the proliferation of apps and developers, these ideas have faded and don't always hold up, but that was the general idea.
Basic commands use the command key with a letter. Adding the shift key usually reverses the command, like cmd-Z in Finder is Undo, cmd-shift-Z is Redo.
The addition of the option key is supposed to add an alternative or extension to the basic command like when you see commands whose text contains an ellipsis (which means that the command will bring up a dialog for more information before executing). Adding the option key can remove the ellipse and just execute the command with the last parameters that were set. In the Finder, cmd-W closes a window, cmd-option-W closes all windows.
Rarely would you find a command that uses all four modifier keys, it's just hard to press them all at the same time.
Of course over the years and the proliferation of apps and developers, these ideas have faded and don't always hold up, but that was the general idea.