Air carrier Captain reported it is unclear if the pavement west of Runway 8R at ELP is designated as a displaced threshold. There are no markings on the pavement or airport chart.

Narrative

We were in ELP taxiing outbound via the ramp towards Taxiway K to hold short Runway 8R Approach at K. When we switched to Tower; we were told Cleared for takeoff Runway 8R at K". In our brief we had talked about how the area to the west of the threshold of 8R isn't marked as a displaced threshold and looks on the airport moving map and 10-9 just like a taxiway; except without a taxiway alphanumeric name. So we had decided it wasn't part of the runway surface and shouldn't be used for takeoff; the way a displaced threshold can be used for takeoff. To make sure we were legal we taxied to the threshold of 8R and departed from there. I'd like some clarification on this. I have several questions. Is the pavement west of the threshold of 8R legal to use for takeoff? If so; why is it legal; since it's not marked as a displaced threshold? What would you call it if it's not a displaced threshold? SAN Runway 27 and SJC Runway 30R both have displaced thresholds that are clearly marked on the Airport Moving Map; the 10-9 and the pavement itself but there are no such markings on ELP 8R. Should there be? This might be a good thing to address on the ELP company information page. Clarification regarding my questions about that area west of the threshold of 8R at ELP and maybe a note in the company information page to prevent similar confusion."