RSET can be a pot, but remember that the pot have to take the same current of the load, and dissipate the consequential power ..... a pot is usually like a 0,25W resistor (some of them are 0,5W, but are not common and easy to find), so when the current go high, it fry and burn in the contact point.
If you need to change the current from specific values, like, as example, 100 and 300 mA, is better that you change the RSET with a resistor, that have in parallel another resistor in serie with the pot, and calculate the values for give to the pot the less possible stress ..... with this type of connection, the first resistor take part of the current and dissipate part of the power, and the second one (the one in serie with the pot) limit the maximum current through the pot and prevent to burn it .....
As example, if i place a resistor of 10 ohm, and in parallel to this i place a serie made with a resistor of 4,7 ohm and a pot of 50 ohm, this give me a total resistance that vary from 8,45 to 3,19 ohm, so i can regulate the current from 147 to 391 mA, and the pot dissipate, at 390 mA, a maximum of 300 mW (also a 250mW trimmer can resist), where instead if you try to set 390 mA witha pot alone, it need to dissipate, at this value, around 450 mW (and it can burn) ..... other than this, a pot alone can go easily in the high current part with fractions of millimeter of movement, increasing the current too much and burning your diode .....
If you need to change the current frequently, or if you need a high current, is better to use this system ..... if instead you need a specific current to set a single time, and the current is low enough, you can also use a single resistor with a trimmer in serie (but it depend from what current you need)
