Unit Properties can store any applescript object by names. Some devices send extra keyed info that is stored here, like low battery flags. Many other datapoints are saved here for you to access and you can store anything specific to the unit that you wish here also. I store the date when I last changed the batteries in a unit in here.
set unit property (text, the key name of the property)
to (anything, the object or string or number or date or anything to store)
in unit (text, the name of the unit in which to store the object. Defaults to the value of thisUnit)
set unit property "last battery change" in unit "kitchen motion" to (current date)