Confused about the centroid of classes?

[Queequeg92]

In your paper, it's said that the centroid of class as average of features over a mini-batch which implies you update the centroid during training according to different mini-batches. But it's also said that "the features are initialized from pretrained models and the initial value of centroid is thereby obtained". As the centroid is determined by a mini-batch, why a initial value is needed?