Does “a” refer to the unit of current? The unit of current is "A".
When a 60V DC generator is operating, the current in the circuit is not only related to the voltage, but also to the resistance of the electrical devices it is connected to, because I=U/ A.
If you are wondering about the maximum current this generator can provide, you can take a look at the W (Watts) rating listed on the generator nameplate, i.e. P=? Then calculate the maximum current value that can be supplied:
I=P/U.
If the resistance of the electrical appliance is too low, causing the actual current to exceed the maximum current that the generator can provide, the generator will be overloaded and easily burned.