iocontrol - interacts with HAL or G-code in userspace
EMCIO = io
EMCIO = iov2
handles I/O tasks like coolant, toolchange, E-stop and lube.
The signals are turned on and off in userspace with G-code
or in the case of E-stop in HAL.
I/O Control V2 (iov2) adds more toolchager support for communication with the toolchanger.
Whether io or iov2 is used can be chosen in the [EMCIO] section of the INI file.
See also http://linuxcnc.org/docs/html/config/iov2.html