If you don't want to have to insert the ".include ....." directive for the .sub file each time you use a model, or have to paste the .sub file contents on the schematic, another option is to open the model's .asy file in LTspice, press Ctrl A to call up the attribute editor, enter the name of the model's .sub file in the ModelFile field, then save the edited .asy file. (This assumes you have the .sub file in the sub folder and the .asy file in the appropriate asy category folder, as mentioned above.)