Section operations on header stack states that
"Intuitively, a header stack can be thought of as a struct containing an ordinary array of headers hs and a counter nextIndex that can be used to simplify the construction of parsers for header stacks, as discussed below. The nextIndex counter is initialized to 0."
The title says it all.
P4 spec (v1.2.3)
Section operations on header stack states that "Intuitively, a header stack can be thought of as a struct containing an ordinary array of headers hs and a counter
nextIndexthat can be used to simplify the construction of parsers for header stacks, as discussed below. ThenextIndexcounter is initialized to 0."