THe PWM pin looks like an enable to me... It's marked enable (EN) However if you read the data sheet carefully there is a missing wire. There is no ground connection on the example schematic for the PWM driver and the LED controller. Left floating it might operate erratically, Connecting to the LED controller will surely be problematical, with out a return (ground) lead. I should think it would go to the PSU ground as the led is likely not grounded. It probably is the current feedback sense for the Constant Current Driver circuitry as it much easier to do "Low Side" sensing than it "High Side"... low will be a couple of volts above ground and that voltage is used to sense the LED current. Otherwise it would be Vsupply - Vsense, varying about a volt or so from Vsupply...