Short answers, pulled from the story.
The Sanskrit word upādāna translates to fuel, material cause, or substrate that keeps an active process energized. Ancient Pali texts define it as the source and means for sustaining a dynamic state.
The Cula-sihanada Sutta identifies four specific categories of clinging found within the Sutta Pitaka. These include sense-pleasure clinging, view clinging, rites-and-rituals clinging, and self-doctrine clinging.
In the twelve-linked chain of Dependent Origination, clinging serves as the ninth causal link. It arises when craving acts as a condition before it can exist.
Professor Richard F. Gombrich linked the literal meaning of fuel to the Buddha's fire metaphor in the Aditta-pariyaya sermon. He taught at the University of London School of Oriental and African Studies during his Numata Visiting Professor Lectures.
The Dhammasangani sections 1213-17 define four types of clinging through mental factors. Sense-pleasure clinging corresponds to the mental factor known as greed or lobha while the other three types correspond to wrong view or ditthi.