The Dimmable type popup changes the behavior of a dimmable unit when using the turn on command.
Simple devices assume that the value goes to full on when an on command is sent. This is the default behavior for legacy X10 units as that is how they physically behave. There are also modern ZWave and UPB modules that behave the same way. If your device always goes to full on when you send the on command (and that is how you want it to behave, see simulated below) then select this unit type
Smart devices are expected to manage their own preset levels. If you turn on a Smart device then XTension will assume it’s going to go to the last preset level that XTension knows about. It may have had it’s own internal level changed manually since then though so the XTension database may assume that the device has gone to 75% but it was changed manually at the switch to 50% and so the database may be wrong in these conditions. The database will be updated the next time the device is queried or changed via a script or direct XTension command.
Simulated devices are where things get more interesting and useful.
If you set a device to simulated then XTension will never actually send an ON command at all but instead will substitute a dim command to the current preset level as known by XTension. You can change the preset level that XTension will send via the sim preset verb.
Most modern devices maintain an internal preset level that is set whenever the device changes it’s level to anything other than off. If you manually dim a ZWave device to 10% then the next time the switch it turned on manually or the next time an ON is sent from XTension it will return to that 10%. So if last night you dimmed your lights but this afternoon you want them to come on at full when XTension turns them on for you you should use a simulated device type or use the dim command rather than the turn on command. Using the sim preset verb in XTension does not send any command to the device itself and will not change the level of the device, it only affects what value is sent with the next on. For currently known units there is no way to send a preset level to a switch without changing it’s current dim level.
In my own home I often manually brighten the bathroom lights to a low value if I am getting up early in the morning. That low level then sticks as the switches local preset and when I tap the switch later they only go on to 10% and I have to stand there and hold the paddle. To work around that at some point after everyone has had their shower and the lights have been turned off I send them a dim to 100 command and then immediately afterwards a turn off command. The ramp rate of the light combined with the fact that LED bulbs don’t turn on instantly means that the lights don’t even flicker on but now the internal preset of the switch has been set to 100% so that when I next turn them on at the switch they go to full on.
Changing a devices value manually at the device does not update the preset level in XTension, so any other ON commands that you send to a simulated device will result in them going to the value that you have set in your scripts.
NOTE: Though Phillips hue bulbs are technically “smart” in XTension they ignore their dimmable setting and are always treated as simulated units. In my testing with them they would often get confused about color or level values, especially in the case of a power hit or other minor such problem. This led them to often being out of sync with XTension. When using Phillips Hue bulbs and sending an ON or a dim command the current color and level information as known by XTension is always sent along with the command.
The 3 types of dimming devices date back to the beginnings of time and the X10 protocol. The Simulated device type was designed to work around the “Nova” feature of early X10 switches and lamp modules. You could not brighten them up from off to a desired level. If you sent them a brighten command they would “nova” to 100% and then from there you would have to send them dim commands to go down to the desired level. XTension introduced the simulated device type so that you could never turn them all the way off. If an X10 device that is set to simulated is turned off the OFF command is not actually sent but instead the device is dimmed to 0 so it appears to be off, but from this state you can say dim to 10 and it will brighten gently to 10% avoiding the “nova” affect of those early devices. Modern devices do not have the nova problem but still benefit from having different ways of handling the preset levels.