question about injector block

[xh-ui]

Dear all,
In the manual of injector block , some phrases are referred, such as "drifting Maxwellian" and “flux Maxwellian” , which comes from the following paragraph:
"EPOCH can inject particles through any of the simulation boundaries. This plasma is either a drifting Maxwellian corresponding to a collisionally thermalized beam or a “flux Maxwellian” corresponding to a Maxwellian source accelerated by an electrostatic accelerator. It can have any temporal or transverse spatial profile of density, temperature or drift that you wish to specify."

I'm really confused about the meaning of the phrases. Are they different in particles flying direction or for other phisical meaning?

And also the parameter of "drift_x", does it mean the momentum distribution ? If it is, it's in FWHM criterion?