what does it mean "using genarators" when training MuseGan ? ( at 7_04 notebook)

[buzem]

In train_critic(self, x_train, batch_size, using_generator):

   if using_generator:
        true_imgs = next(x_train)[0]
        if true_imgs.shape[0] != batch_size:
            true_imgs = next(x_train)[0]
    else:
        idx = np.random.randint(0, x_train.shape[0], batch_size)
        true_imgs = x_train[idx]