If you have ever changeda System Variable in AutoCAD, I bet that you have wanted to know what options were available. Well, There is tool found at:
Express Tools Tab > Tools panel > System Variables
Or the command = SYSVDLG <enter>
System Variable Dialog
This dialog box shows the system variables that you can change and gives a description of the system variable. And most importantly, it shows the available options for the variable and describes each option.