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The standard Laser scan message describes only a single returned depth and intensity value for each laser pulse, this is usually the most intense if there are several but this depends on your sensor hardware.

Multi-echo sensors are able to return several echos for each pulse emitted by the laser. For example if you scan through a window you will often pickup an echo from the glass as well as the wall behind it, or at the boundary of two objects at different depths you can receive two echo's for the depth of each object. This additional information gives you more points which can be used to create your map and localise the robot. So a node that receives multi echo laser scan messages can use this type of sensors to their full potential.

Hope this helps.