When the Frames and Arguments preference has the value Tree-view , the Debugger appears as shown in Variables in the Debugger tree view below.
Each expandable root node in the Backtrace: tree represents a stack frame associated with a function call. You can operate on the frame as described in Backtrace area.
Expanding a stack frame node displays any variables associated with that function call. You can double click on any variable to inspect it using the Inspector tool. Any items selected in this area can be operated on using the commands in the Variables menu: see Performing operations on frame variables for details.
Figure 11.2 Variables in the Debugger tree view
Each call frame is a root in the tree with a 
icon and has several kinds of subnode:
icon represents a closure variable (either from an outer scope or used by an inner scope).
Double-click on a 
icon to show the source of that function, if available, in the Editor. Double-click on any of the disc icons to show that variable in the Inspector.
LispWorks IDE User Guide (Macintosh version) - 25 Nov 2011
icon represents a normal lexical variable. 
icon represents a special variable.
icon represents some other frame.