A voltage generator

The voltage generator is a device that can create a potential difference between the positive pole and the negative pole.

Generated voltage can be of two types that you can see the symbols below:

Alternate Current
Continuous Current

Continuous current is for example that generated by a battery so that you can locate the "positive" pole, that is the one with the greatest potential.

The alternating current (that of our homes at 220-230 volt) instead inverses its polarity continuously, it does not make sense to mark the positive pole.