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From manual, page 22:

Important note: IES and Eulumdat files are commonly used to mimic lighting fixtures,
using the lighting information files provided by manufacturers. They are handy because they
can easily imitate the complex lighting effects produced by certain specific fixtures, without
having to model the lamp itself. However, they must be used with care because they can
sometimes produce physically incorrect results.
Both formats contain the luminous intensity at any angle, measured in laboratory using light
sensors located all over a spherical dome around the light source, pointing to the center of
the dome so that the data capture method considers all lamps as point sources. This means
that with these formats, the physical correctness of the result is highly dependent on the
geometry you apply the IES/ Eulumdat files to.
To get the most physically correct results, IES/ Eulumdat files should be applied to small
spheres to copy the laboratory environment they were measured in as much as possible.
Applying an IES or Eulumdat file to a geometry other than a small sphere or using files
captured from a non-point lamp can cause inaccuracies inherent to the method and format
definition.