Whether the "sending unit" is running a gauge or an indicator lamp, it's sending information in two different manners. A "sensor" uses varying voltage to make the gauge needle more in the appropriate direction for the amount of voltage it's sending through the circuit. A "switch" is either "on" or "off", hence it is used for an indicator lamp that's either lit or dark (i.e., temperature warning lamps, "cold"/"hot" light displays, oil pressure warning lamp)
In the case of the Imperials with the gauge AND indicator lamp, there should be an oil pressure sensor and an oil pressure switch on the back of the block, each with different circuits and functions, even if they might screw into a common fitting on the back of the block.
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