Usually, in the case of a battery, its the voltage printed on the battery. A 12 volt SLA, when charged, will give about 14.5 volts, and a badly overcharged one somewhat higher, for a time. Any circuit designed for use with that type of battery should take this into account. It should also take into account the fact that during discharge, the battery voltage will drop below 12, so the circuit should neither cook with the higher voltage nor fail to operate with the lower.
Any reference to a non-battery supply at other than a recognizable battery voltage, is shown as "nominal", should also declare its hand by stating what its limits are. A battery eliminator may show a "nominal" voltage output. This will be load dependent unless the eliminator is voltage regulated, and unloaded my exceed the printed number considerably - under load it will drop.