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There are two main types of tracers in fluids:

  1. Passive: a tracer which does not effect the flow.
  2. Active: a tracer which does effect the flow.

Imagine someone thrown into the Atlantic ocean. No matter how hard they swim they do not change the gulfstream flow in any substantial way, so we consider their motion inconsequential. Mathematically the assumption here is that both the size and inertia of the tracer, which would effect the flow if they were large enough, may be neglected. On the other hand potential temperature and potential vorticity are active tracers, and their effects must be taken into account.