A 12 and 14 volt alternator are the same thing. Alternators need to produce more than 12 volts if they are going to be used in a 12 volt battery system. The actual voltage regulator set-point is typically between 14.1 and 14.8 volts and that is why an alternator may be referred to as a 14 volt alternator instead of a 12 volt alternator. The battery needs to receive 14+ volts in order to maintain its charge.
The same rules apply for a 24 volt system. The actual voltage regulator set-point is typically between 28.1 and 28.6 volts and that is why an alternator may be referred to as a 28 volt alternator instead of a 24 volt alternator. The battery bank needs to receive 28+ volts in order to maintain its charge.