difference between 'fpt.V' and 'fpt.vx'

Hi everyone:
I use the fluid particle tracing module(fpt) to calculate the trace.
I want to know the differences between the 'fpt.V'(particle belocity magnitude) and the fpt.vx,fpt.vy,fpt.vz(particle velocity,x,y,z component) .
AND also what is the difference between the particle velocity and the negative particle velocity?


In the fpt module,I added no force but the default color expression showed that the 'fpt.V'(particle belocity magnitude) is 14.901e-9. It puzzled me.

Best regards!