The naming rule of ResNet as follows:
stage 2 and stage 5 both have 3 layers, so they are named as ‘a’, ‘b’, and ‘c’.
stage 3 and stage 4 has more than 3 layers. And there are clear differences between the first layer and rest layers of those two stages. So, their first layer is named as ‘a’, and rest layers are all 'b’s. In order to differentiate those 'b’s, they are named as ‘b1’, ‘b2’, and so on. Hence, for ResNet-101, stage 4 has 23 layers. They are one ‘a’ layer and 22 ‘b’ layers, ranging from ‘b1’ to ‘b22’.