Plain-language answer: The far field, or Fraunhofer region, is where the wave is locally transverse and the normalized angular pattern is essentially independent of distance.
Why it changes the path
Gain, polarization, beamwidth, and inverse-distance field relationships are most directly applied there, but required distance grows with antenna size and frequency.
What to observe
Calculate a suitable minimum range for the largest antenna dimension and verify unobstructed geometry and polarization.
The common mistake
Using far-field equations inside the reactive or Fresnel region.
Planning boundary
Propagation models and field regions depend on frequency, geometry, terrain, atmosphere, polarization, antenna pattern, and measurement distance. Use them to plan and compare—not to guarantee a contact or installed range.
