A bus selector chooses some subset of bits in a bus as a new bus. This is useful for selecting fields from a value. For example, the biased exponent in a half-float value is contained in bits 10 to 14. If you drag in a Bus Selector component and double-click, you can set the bit range from 10 to 14 and it will extract the exponent into it's own bus:

The output is a bus if more than one bit is selected and is a bit if only one bit is selected.

That would be equivalent to expanding the bus to single bits, selecting the desired bits, and making them back into a bus: