Martin (1992: 112-3):
The general phoric question word involving reference to a superset is which. Which questions are responded to either by using an Epithet or Numerative to select the relevant member of the superset or by using a Deictic that says it doesn't really matter. The opposition is illustrated in [3:34]:
[3:34] Which one would you like?NON-SELECTIVE — Both/either/neither/each.SELECTIVE — The first one/the next one/the biggest one etc.
The [non-selective] Deictics distinguish between individuated (each) and dual (both, either, neither) reference. Within dual, inclusive both contrasts with alternative either/neither.
 Here again the (interpersonal) deictic function of determiners in nominal group structure is mistaken for the (textual) reference function of determiners in cohesion, and the grammatical confusion is rebranded as a set of discourse semantic distinctions. The theoretical inconsistencies (misunderstandings) are thus both metafunctional and stratal.
 Here again the experiential construal of participants by nominal groups at the level of grammar is mistaken for textual reference at the level of discourse semantics. The theoretical inconsistencies (misunderstandings) are thus both metafunctional and stratal.
 To be clear, the only Deictic that "says it doesn't really matter" is either. This is not true of the other members of this set (both, neither, each) and so, does not identify even the deictic function of these determiners.