The grid_type resource is used to specify the specific grid type number from the GRIB header.  In some cases, GRIB files will contain grids from more than on grid type.  This way, you can specify only grids from that type. In this resource is not specified, all grids are parsed.

If the grid type is unknown, it can be obtained using the griblook program:

griblook -cu -mo=eta -me=out4

      GRID FILE PARSER (Ver 5.0-X11)

Current filename: /home/wxp/hds/97092900_eta.grb
Reading from grid file:/home/wxp/hds/97092900_eta.grb
  0: ETA analysis for 0000Z 29 SEP 97-100 mb Vertical velocity (Pa/s)
     Model: ETA  Time: init         Level: 100mb        Variable: vvel    Units: Pa/s
     Model: 89 Ltype: 100 Level: 100 Var: 39 Hour: 0 Grid: 211
     Proj: 4 Lat: 40.62 Lon: -100.51 NX: 93 NY: 65 DX: 0.63 DY: 0.63
     lamb::-95.000:25.000:25.000:1.277,40.620,-100.510,93,65,0.6263,0.6263

The grid type in this case is 211 and is listed on the second line of the description after the keyword "GRID:".