West Coast.
I think you may be confusing the 'Display Volume' with the 'Surveillance Volume'.
Surveillance Volume:
The range that TCAS can track other aircraft. In range terms this is variable and depends on transmission power (which in turn is affected by traffic density), but is usually around 40 NM. In altitude terms is is only limited by the maximum / minimum limits of the encoding in Altitude Reporting - something like -1,000' absolute to +100,000' or more. For a precise limit, ask and avionic expert, but effectively unlimited.
Display volume:
The volume that is shown on your traffic display, which typically +2700' to -2,700' relative, extendable in either direction up to around 8,000' to 9900' (type dependant), out to whatever range is selected on the traffic display / EHSI. Range settings anywhere between 6 NM and 40NM are common.
The display volume only determines whether or not 'Other Traffic' blips (usually open diamonds) are shown.
All other categories of blip, be they Proximate, TA or RA are always shown ,regardless of Display Volume selection.
i.e. TCAS tracking and collison avoidance assessment is not affected, in any way, by the display volume settings.
If a TA or RA is in progress, but outside your vertical settings, it appears anyway (note - proximate traffic criteris means they are always within relative altitude range).
If a TA, RA or Proximate is in progress, but outside the range, it appears as a half blip at the edge of the display.
Lt Dan,
Against non altitude reporters, TCAS assumes the worst and that all vertical triggers are met. Hence a TA is generated. Obviously, since there is no vertical data, an RA can not be issued.
The associated blip is still shown, albeit minus altitude tag, to allow the crew to see what direction to look in.
To be honest, Non altitude TAs are quite common. I'm surprised the crew mentioned it.
Some TCAS types have an inhibition of non altitude TAs at high altitude. Rationale is: non altitude = high probability GA traffic below CAS, therefore ignore. Typically cuts in at the high teens of thousands of feet.
Hope thats of some use.
CPB