What is "Z Direction" in Force field? Hi. I'm doing some particles following guided line work.
And I was wondering the function of "Z Direction" in force field which is applied to curve(=guided line).

What is the exact function of z direction? I assume that might be the direction of the force.
But when I change the direction from +z, -z, I don't know exact difference there.

Any answer would be great. I'm so curious about that but no one could answer me...
it bothers me.. a lot....

Thanks.
The Z Direction setting is within the Falloff section of the Force Field settings and as such it defines how the force reduces in strength as you move along the z-axis. With it set to 'Both' it will decay in both directions, set to +Z in the positive direction only and with -Z in the negative direction only. 

Note also that the rate of falloff is determined by the 'Power' - with the Power set to 0 the force will always be distance^0 which will equate to '1' and so no actual falloff regardless of your Z Direction setting. Use a power of '1' for linear falloff, '2' for exponential fall off (such as with natural forces, gravity, etc.).