Spiritual master means they must have many followers. They are all Vaisnavas. They are called prabhus, and the spiritual master is called prabhupada because on his lotus feet there are many prabhus. Pada means lotus foot. So all these Vaisnavas, they are all prabhus. So they are also offered respectful Not that the spiritual master alone, but along with his associates. And these associates are all Vaisnavas, his disciples. They are also devotees of the Lord; therefore they should also be offered respectful obeisances. This is the process
We are taught to address others as Prabhu. Prabhu means master; and the leader of the masters is called Prabhupada. So if the Prabhus have surrendered to the Prabhupada.
The prakrta-sahajiyas are not even worthy of being called Vaisnavas. They think that only caste gosvamis should be called Prabhupada. Such ignorant sahajiyas call themselves vaisnava-dasa-anudasa, which means the servant of the servant of the Vaisnavas. However, they are opposed to addressing a pure Vaisnava as Prabhupada.