Intermediate format data may be on constatnt pressure levels (most common), and on some height surfaces. If your data are on pressure levels, you will need to have the matching height data on the pressure levels. If your have data on height levels, you will need to have the matching pressure fields on those height levels. This will ensure that you can do interpolation to the vertical grid that the model uses. For pressure level data, the pressure value, itself, is provided in the 'header' written before the 2nd slab, which is the variable xlvl in the format description in the WPS chapter of the WRF Users' Guide - see Writing Meteorological Data to the Intermediate Format