In Gaudīya Vaishnavism the Sātvata-tantra describes three different forms of Vishnu as: Mahā Vishnu, Garbhodakaśāyī Vishnu and Ksirodakaśāyī Vishnu (Paramātmā). Each form has a different role in the maintenance of the Universe and its inhabitants.
Vishnu’s expansions are literally innumerable. But they are non different. Among these expansions- Maha-Vishnu is one and only one who creates innumerable universes. After creating them, He manifests within these innumerabe universes (as Garbhodakshayi Vishnu) and also within each being and within each atom. (as Kshirodakshayi Vishnu). There is one more expansion which is not in this material world but in the spiritual world who is known as Narayana or Vaikuntha Narayana. It is said - “Viṣṇu has three forms within these material universes called puruṣhas. The first, Mahā-Viṣṇu (Karnodakashayi Vishnu), is the creator of the total material energy [mahat], the second is Garbhodaśāyī, who is situated within each universe, and the third is Kṣīrodaśāyī, who lives in the heart of every living being. He who knows these three becomes liberated from the clutches of māyā.”
Maha vishnu or Karanmodakshayi Vishnu :
Mahavishnu or Karanmodakshayi Vishnu is situated in Karanamodaka Ocean as suggested in the name itself. It is the causal Ocean that is the Ocean which is cause of the creation. This ocean is within the material world in which there are many material universes. Maha-vishnu is outside these material universes but within the material world. It is difficult for me to state this in written format. Please refer the images.
He lies within the Causal Ocean and breathes out innumerable universes. When he exhales, all the universes come out from the pores of His body just like the bubbles come out of the water. When he inhales, all the universes enter within Him. This is universal inhalation or massive destruction. Such universal eradication only takes place when all the gross elements are transformed into their void state known as Mahad tatva. This happens after Sada-shiva evaporates the gross elements through the heat of his third eye. There is also partial inhalation in which half of the part of material universe is inhaled that is, destroyed. In short, Maha Vishnu is cause of the creation of innumerable material universes and their inhalation.
Garbhodakshayi Vishnu :
Garbhodakshayi Vishnu is situated in Garbhodaka-Ocean. It is the Ocean which is womb of beings within this material universe. Maha Vishnu explained above, after creating innumerable universes from the pores of His body, manifests within each universe as Garbhodakshayi-Vishnu.
There are innumerable universes thus innumerable Garbhodakshayi Vishnus. From Garbhodakshayi Vishnu’s navel comes the Lotus which rises up to BrahmaLoka after which it opens and Lord Brahma is born. As there are uncountable universes, there are uncountable Brahmas.
Kshirodakshayi Vishnu :
Just like Garbhodakshayi Vishnu is situated in each universe, Kshirodakshayi Vishnu is also situated within the universe in the ocean of Kshira. Kshriodaka-Ocean literally means- the ocean of milk. It is not the actual milk which we see and drink but it’s a celestial and etherical fluid which looks like milk.
Khrirodakshayi Vishnu manifests within each heart as Paramatma. Paramatma and atma are different as the Upanishads say about the two birds, one eats the fruit (soul who enjoys the fruitive actions) and other keeps seeing the other bird (Supersoul who is Kshirodakshayi Vishnu explained here who examines our actions and gives the suitable reactions). This feature of Vishnu is also present within the atom, within each smallest particle of material elements. The protons, neutrons and electrons are formed because of the intention and effect of Kshriodakshayi Vishnu.