A sub circuit is a device which can contain a separated new circuit.
Choose this device type from the menu.
Drop a new sub circuit and double-click on it and a new and empty window opens.
You can create a new circuit in this window now.
To go back to the parent circuit, use the blue arrow on the left upper side of the window. If you are on top level of all circuits, the blue arrow goes hidden.
Define an interface between the parent circuit and the sub circuit by using the special devices "input" and "output".
These devices have no delay, signals will going directly to the devices connected to them.
If you have defined some inputs or outputs and you go up to the parent circuit using the blue arrow,
the names of the interfaces will be shown as a part of the symbol of the sub circuit.
You can connect these interfaces as usual now.
To change the name of an interface device, simply double-click on it (being in the sub circuit) or use the menu which appears when right-clicking such a device.
Unless you havn´t defined some interface devices in the sub circuit, you will not be able to connect it to other devices (being in the parent circuit).
You can save a sub circuit as it is, open it and choose "File/Save".
You will be asked which type of file you want to create.
If you have saved a sub circuit as a sub circuit, you can reload it any time and anywhere without destroying the other parts of your circuit.
To display a sub circuit in a separate window, right-click on it (in the parent circuit) and choose "open in window".
You can use all usual operations in the new window.