Self custody in crypto is a situation where only an individual have the possession of his/her digital assets because he controls the private key. In this situation the individual is fully responsible to safeguard access to his/her private key. Below I shared a link of a reddit write up shared by founder of Ethereum Vitalik about self custody https://old.reddit.com/r/ethereum/comments/11tijiv/how_i_think_about_choosing_guardians_for_multisig/