What is select line in multiplexer?

Could you please clarify what exactly you mean by "select line in multiplexer"? As I understand, a multiplexer, or MUX, is a device that selects one of several analog or digital input signals and forwards the selected input into a single output line. The selection is typically controlled by a set of input lines, often referred to as select lines or control lines. These select lines determine which input signal is routed to the output. Is that what you're referring to, or do you have a more specific aspect of the select line in a multiplexer that you'd like me to explain?