To add to the above, it was actually first introduced in Groups and Scenes, then later added to RM per some user requests. (BTW, if you are on a C-7 and were doing it to make Z-Wave happier, this should be less necessary, or perhaps not even at all, after the optional-but-highly-recommended Z-Wave firmware update in 2.3.1, BTW; see here for more on that: https://community.hubitat.com/t/c7-2-3-1-and-z-wave-firmware-7-17-1/.)
It is documented for Groups and Scenes, and the purpose and outcome is the same for Rule Machine. As noted elsewhere, it is available only for "Custom Action."