Hello Terry, this is a great question, thanks for asking Galco today.
The PUT is a Programmable Unijunction Transistor where the UJT is a Unijunction Transistor.
The UJT has what is called an Intrinsic Standoff Ratio that only let’s this device conduct (or switch states off to on) at 1 specific voltage level.
The PUT is called programmable but that only means you can scale where it fires or changes state to conducting. By simply putting a voltage divider on the Gate of the PUT you can easily shift the point of conduction with respect to the Anode/Cathode terminal voltage. These devices were commonly used in SCR Gate firing circuits to simplify circuit design. With a DC control voltage and an AC sync voltage a PUT and an amplifier to drive the gate pulse transformers your circuit was complete.