Automatic Symbol Generation

A symbol can be automatically generated in two situations.

When editing a schematic, you can execute menu item (Right click menu)=>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 in 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." For most users, this is the only method you should consider for adding third-party models defined as subcircuits since all the details are handled for you.