Car battery specifications relate to case size, design and current capability, measured in cold cranking amps. Another specification of car batteries is also reserve capacity. Case size and design are the most important specification for a battery because this is the size and shape of the battery tray it will fit into. As well as location and type of battery terminals. Cold cranking amps are the next most important specification of a car battery. This is because a battery may fit a battery tray, but doesn’t supply enough current to start the engine, making it worthless. Reserve capacity describes how long a battery can maintain a useful voltage when being discharged at a 25 Amp load.
For more info, see Does Car Battery Size Matter