Different marks, Tilaks
The Tilak spot is consciously placed at the center of the forehead between the eyebrows. This location is significant being the location of ajna chakra, which is also known as the third eye, the eye of wisdom (jnana chakshu) or the Divine eye.
Followers of Vaishnavism mark their foreheads with different styles of tilak to show that they are servants of Vishnu. The markings are made either as a daily ritual, or on special occasions, and denote which particular lineage, or sampradaya the devotee belongs to. The different Vaishnava sampradayas each have their own distinctive style of tilak based on the siddhanta of their particular lineage. The general tilak pattern is of two or more vertical lines resembling the letter U, which commonly represents the foot of Vishnu.
In the Vallabha, Rudra-sampradaya the tilak worn is a double vertical red line which is rounded from base. This "U" represents Purna Purushottama Shri Krishna's lotus feet who is the only form of god worshipped in the Vallabha sampradaya.
In Nimbarka Sampradaya, the tilak is made of Gopi-Chandana (the clay from Gopi Kunda lake in Dwarka, Gujarat), as described in the Vasudeva Upanishad. It starts at the bridge of the nose and continues as two vertical lines to the top of the forehead. This is said to represent the temple of God. Within these lines, between the eyebrows is a black dot, made from the slate found in Barsana, Uttar Pradesh, the sacred birthplace of Radha. This is said to represent God as Radha and Krishna together. This tilak personifies the tenets of the Sampradaya, that God is Radha and Krishna together, none else. It is supposed to have been first given to Nimbarka at the time of his initiation to the sage, Narada. The tilak is first given to an initiate by their guru at the time of initiation, and after this, daily the devotee will remember his guru before he adorns the tilaka on his head.
The tilak is made of sandalwood paste. This has a cool, soothing effect and the fragrance helps to maintain a pure and clear mind. The chandlo is made of red kum kum (potash nitrate).The tilak represents the Lord and the chandlo represents the muktas that reside within Him. Once the Muktas enter the murti, they will eternally remain there. There is a union between them, i.e. they appear as one, but there is a distinction, the master and the devotee. When the two merge, they appear as one, but in reality they remain distinct in their nature and their attributes, with God remaining the master and the soul forever remaining the servant.