Basically, the FumeFX Follow Operator forces particles to move in the same way as fluid does throughout the simulation. This will makes it seem as though the particles are part of the fluid, and can be used to add detail to the simulation. This operator only acts on particles that are inside the fluid - all other particles are ignored. To be able to create it, you need to have FumeFX and particle shape selected.
Assigned to - It will display to which FumeFX it is attached.
If you change the Velocity Magnitude from the default value of 1, particles will respectively faster or slower than the fluid, but still in the same direction. Negative values are supported, so particles can move in opposition to the fluid.
If enabled, FumeFX will create additional per - particle attributes.
If a particle leaves the FumeFX grid - whether permanently or temporarily - you can choose to delete it by turning on the If Out Of Grid checkbox. Likewise you can also delete particles that enter areas in simulation that are occupied by an object by activating the In Contact With Object checkbox. If the particle is not deleted it will continue its movement uninterrupted by FumeFX.
Note: Particles can be deleted only if their lifespan is set to "lifespanPP" mode, as shown on the image below.