I have a drawing and there is a feature in it which is named DISTANCE_EXP(29). What kind of feature is this, how do you have to use it. I cannot not find any documentation in UGNX3.
It is an associative distance expression. Someone took the distance between 2 entities and saved the value. As the model changes this distance expression will update (as long as the entities that the distance expression rely on are not deleted). The distance can now be used to drive parameters of other features down the tree. If it doesn't have any dependents (child features) it can be deleted (though you may lose some design intent).
For more info try searching the help files on 'distance expression' and/or 'expression'.