Where this mechanism sits in the scope's delivery chain: 0 is the primary, 1 the first fallback, and so on.

On a declare, an absent value means 0 — there is no prior position to preserve. On a replace, an absent value leaves the position unchanged; it is never defaulted to 0, because doing so would silently promote every edited provider to primary.

A value supplied on a replace is applied with insertion + renumber semantics: the provider is inserted at that index of the scope's chain for this channel and the remaining providers keep their relative order, renumbered contiguously from 0 in one transaction. A value deeper than the chain places the provider last.