In this context, a multi endpoint device is a single device that has more than one actionable part. So it might be a key fob with three buttons. It might be a wall switch, like the new Aqara model, which is a double rocker, so essentially A and B but with a single device address on the network. both Zwave and zigbee refer to these as “endpoints.”
There are lots of different styles of these.
Internally, smartthings tends to treat these as “buttons.“. (Or as one button but with many different tap patterns, like saying that the top of the right hand rocker is a triple tap on one button. It isn’t, but that’s how SmartThings will handle it.)
Another problem is that the way in which multi endpoint Devices were handled in SmartThings has changed over time, And to be honest the platform was never great at them anyway.
The most common symptom you run into is that only one switch on the double rocker will work in smartthings. Or only one button of a multi button switch. So out of the box the device will add to smartthings and you will be able to control The first button/rocker, but the other endpoints will be invisible to smartthings.
There has been much discussion of this in the forums, as was mentioned, and the usual fix is new custom code that will create new constructs that will enable smartthings to work with the additional endpoints.
Zigbee also allows for the use of multiple endpoints on a single device to handle different kinds of situations, such as having one endpoint for a ZHA profile and another endpoint for a ZLL profile. (The hue dimmer switch used to do this before the Zigbee 3.0 version.) or you might have a single device which has multiple functions, like a thermostat which has a temperature sensor on one endpoint and a power control on another endpoint. For anybody who’s interested in that technical aspect of multi endpoint Devices, there are lots of references, such as the following:
But in a smartthings context, it usually comes up because of a multi button device where only one button works out of the box.