Subcircuits are ordinary JLS circuits that are imported into other circuits. The imported circuits use input and output pin elements to allow them to be connected to wires in the circuit they are imported into.
Circuits that are currently open in JLS (i.e., have a tab showing) can be imported into the currently visible circuit by pulling down the import list at the far right of the toolbar, then choosing and placing the desired subcircuit.
To import a circuit that is not open in JLS (i.e., a circuit file), use the import menu item in the File menu.
Give the new subcircuit a name, then click on OK to begin placing the subcircuit element, or on Cancel to cancel it.
The subcircuit just imported will be a copy of the original circuit. Hence any changes made to the copy will not be reflected in the original circuit, nor in any other imported copies of the circuit.