jit.unpack 1 @jump 3

Hi I am just trying to make sure I am getting what this means. it is coming from a jit.gl.gridshape. since gridshape can give 12 planes, and I only need the first 3, this is apparently the syntax : "jit.unpack 1 @jump 3" . so why is it not jit.unpack 3? I guess all three of the planes are essentially not being unpacked but instead separated as a unit from the others and coming out as one, as if already packed? am I to assume jump means to take the argument number and move that many out?