Mercenary type: fanatic, berserker

In the hirelings section about mercenaries, there is a table for morale which lists a category naming fanatics and berserkers which receive the highest possible morale bonus. While all other types of Merc are described on the following table by the type of gear they have, fanatics and berserkers receive no description about how to determine whether a Merc is one of these types. What exactly qualifies a Merc as these types?