Tapa is part of the Panchasamskara ritual and is adopted to
make a devotee eligible for rendering service to Lord Vishnu. Panchasamskara
ritual is mentioned in the Pancharatra Agamas.
Tapa refers to the marks on each shoulder of the initiate
made by the acharya by putting heated metalmark in the form of Vishnu’s discus
(Sudarshana Chakra) on the right shoulder and another in the form of His conch
(Shankh) on the left shoulder.
The scars made by these marking insignia unmistakably
distinguish Srivaishnavas from the devotees of all other cults.