Slitherine

I have noticed that some units (ie universal carrier) have ap ratings while in gameplay. When i try to attack with them though, the game says that the unit has no AT charges or is out of range. What does this mean?
Much appreciated

Infantry units use AT charges to attack armor
The number of AT charges is shown by the icon under the bullet

Oh yeah I think some units have a AP rating but the range of their AP attack is 0