From the LTspiceHelp in version 4.01r;
   Schematic Capture > Creating New Symbols > Adding Attributes section;

There is a special combination of attributes that will cause a required library to be automatically included in every schematic that uses the symbol:

Prefix: X
SpiceModel: <name of file including the spicemodel>
Value: <What ever you want visible on the schematic>
Value2: <The value as you want in the netlist>

Value2 would be made to coincide with a subcircuit name defined in the file including the spicemodel and may pass additional parameters to the subcircuit. When a symbol is defined in this manner, an instance of the symbol as a component on a schematic cannot be edited to have different attributes.

If you wish the symbol to represent another page of a hierarchical schematic, all attributes should be left blank the symbol type should be changed from "Cell" to "Block". No attribute values need be set.

There is a symbol attribute, ModelFile, that may be specified. This is used for the name of a file to be included in the netlist as a library. See the symbol/subcircuit pair .\lib\sym\Opamps\1pole.asy and .\lib\sub\1pole.sub to see an example of the utility of this attribute. If the prefix attribute is 'X' and there is a symbol attribute SpiceModel defined that is subcircuit defined in the model file, then a drop list of all subcircuits names will be available when an instance of the symbol is edited on a schematic.
