"Obsession" features a setting called "disrupt-B" or "disrupter-B"...
...And no episode features any sort of a limit on how many settings the TOS weapons might have had, or which sort.
Probably there are at least two and perhaps three variables that may be adjusted to create the desired effects. One might ramp up the total power to kill a tough opponent, but it might be easier to just increase the degree of phase disruption, and to set the stun charge to zero because it won't add anything to the mix. In order to stun the opponent, one might ramp down the power - or then slide power and stun charge to full and completely turn off phase disruption. Etc.
Depends a bit on how you see the phasers working. But only two props appear to have had the sort of switches and toggles that we might readily associate with specific functions: the ENT phaser and the STXI phaser have a special button for flipping from stun to kill and back. In all the others, it's a combination of nondescript buttons, rollers, slides or switches, with or without some sort of a display that tells the uninitiated very little.
In my personal little delusionverse, phasers are basically weaponized transporters - both are based on the same fictional "phasing" phenomenon that removes matter from this reality and takes it to another, thereby cleverly circumventing several laws of physics. The transporter-like nature of the beam also allows it to deliver physical substances, such as nanoprobes in VOY "Microcosm" or Jem'Hadar poisons in DS9. Stun effect might thus simply be the transportation of powerful sedatives to the victim, or then the transportation of a debilitating electric charge. One can then choose whether to transport these stunning elements in the beam, and in what quantity; or one can transport "sheer power" that creates heat in the target. Or one can settle for transporting out chunks of the victim, or the entire victim.
Timo Saloniemi