Effects of Morale and Morale Modifiers

In the export_descr_unit file the base morale values for units can be easily enough seen and edited. But how does this value actually work? For example, if a unit has 12 morale, and is charged by heavy cavalry, will thats units current value decrease say to 10, or does the value of 12 effect how likely a unit is to rout upon being charged, say a 20% chance? Also, are there clear and editable values for morale modifiers such as gunpowder weapons (say -2 morale per volley), or Carrocio Standards (say +4 per standard)?