On the neoStat, you have the following functions
Standby You activate by navigating to the power icon and pressing tick. In this mode, the thermostat will maintain the Away Temperature (normally 12C, but you can change that from 7-17C - read here to learn how)
Temperature Control You can use the Up/Down keys to adjust the desired temperature. The thermostat will always follow its programmed temperature but you can use these keys to temporarily override this.
In HomeKit, you have the same control. The switch directly controls the Standby function, allowing you to turn off the heating when you leave home as part of a home automation rule. Like on the neoStat, you can also override the temperature and you can use automation rules to set a temperature when you leave or arrive home.