Function read_axis_for 

pub fn read_axis_for(axis: RankedAxis, index: &Index) -> RankedAxis

The axis whose secondary a ranked or having read of index walks for a request on axis: the requested one, except that a document count (Count) over a summableOffCountIndex index walks the Sum secondary. Such an index’s counters each count one group in its count trees and add their group’s documents to its sums, so its sums are its document counts (document_count_of_element).

Unversioned, so every protocol version reaches it: it departs from the plain axis only on a summableOffCountIndex index, which only meta-schema v3 (protocol version 14) admits.