Question on use of ZDEF component

[Tero Siili]

Dear colleagues,

the GrADS documentation on ZDEF (http://www.iges.org/grads/gadoc/
descriptorfile.html#ZDEF) says:

> The LEVELS mapping method requires one additional argument, value-
> list, which explicitly specifies the pressure level for each grid
> point in ascending order. value-list should contain znum floating
> point values. It may continue into the next record in the
> descriptor file, but note that records may not have more than 255
> characters.

My question is, does the vertical coordinate HAVE to be pressure or
does "pressure" above mean "any reasonable vertical coordinate" ---
such as sigma?

Tero Siili