  The large scale fluctuations of the ordered state in active matter systems
are usually characterised by studying the "giant number fluctuations" of
particles in any finite volume, as compared to the expectations from the
central limit theorem. However, in ordering systems, the fluctuations in
density ordering are often captured through their structure functions deviating
from Porod law. In this paper we study the relationship between giant number
fluctuations and structure functions, for different models of active matter as
well as other non-equilibrium systems. A unified picture emerges, with
different models falling in four distinct classes depending on the nature of
their structure functions. For one class, we show that experimentalists may
find Porod law violation, by measuring subleading corrections to the number
fluctuations.
