Vorticity expressed as a scalar field when using the ParticleTracer filter

Hi,
I am using the ParticleTracer filter which has a “Compute Vorticity” option. I was wondering why the generated vorticity field is a scalar and not a vector field. Does this vorticity correspond to the projection of the vorticity along the selected input vectors (e.g. velocity) or is it the magnitude of the vorticity?
Thanks in advance,