When does someone receive the Kāma seal?
Kāma attribution is not automatic, but a conscious act from an Otsutsuki.

As of now, including the Boruto manga, we have been able to observe 3 different Kāmas which I will try to explain without spoiling the scenario. Be aware, however, that for the 3rd Kāma, I will delve into spoiler territory as I will need to talk about a character which has not been shown yet in the Boruto anime.


Boruto : Momoshiki implanted the Kāma on his hand by projecting his soul and chakra, and willingly marking him as a Kāma vessel. 
Kawaki : A teenager who has been extensively experimented upon by Jigen and his cronies. Kawaki received a copy of Jigen's very own Kāma through scientific means and chakra transfer, which is a completely intentional transfer.


Jigen :


  3. Jigen : A man of unknown origins. His Kāma (identical to Kawaki's) stems from Isshiki Otsutsuki, Kaguya Otsutsuki's consort. He is a perfect vessel, meaning his personality has now been definitely erased by Isshiki and solely houses the Otsutsuki. Isshiki seeks Kawaki's body because he also is a perfect vessel.




The word "vessel" is crucial, because the main purpose of the Kāma is to create a literal vessel for the Otsutsuki soul which created the Kāma (as confirmed by Jigen himself). It is unconfirmed for now, but Momoshiki could try to take-over Boruto's body later on. We know that Otsutsukis can reincarnate after death, or at the very least transmit their power to individuals they are related to : Naruto and Sasuke were the "transmigrants" of Ashura and Indra, semi-reincarnations influenced by their ancestors. The Kāma bonded Boruto to Momoshiki in death like Ashura and Indra are linked to Naruto and Sasuke by blood.

Tl;dr Kāmas are granted by Otsutsukis who need to hijack a new body