• The actual port name. For example, a 2x2 coupler can have the following terms: ‘in1’, ‘in2’, ‘out1’, ‘out2’. In this case, port_labels = ['in1', 'in2'] would only affect the input ports of the coupler.
• Specified by a direction. For example, “E0” (east 0), “E1” (east 1). Counting starts bottom-up. For example, port_labels = ['E0', 'E1'] will select the two lowest east ports (after applying transformations onto the contents, given by content_transformation).