A symbol can be automatically generated in two situations.
When editing a schematic, you can execute menu item Hierarchy=>Open this Sheet's Symbol. When no symbol is found, LTspice will ask if you would like one automatically generated. This symbol then can be used to call this sheet of circuitry is some higher level schematic.
Also, when editing an ASCII netlist that contains subcircuit definitions, you place the cursor on the line containing the name of the subcircuit, right click, and execute context menu item "Create Symbol"