Distribution functions in polymers are discussed. For a more accurate modelling of phenomena in physics or materials, especially polymers and composite materials made of elements of varying types or sizes, an approach slightly more complex than mere algebraic base formulas is required. This involves considering the composition of various material fractions, known as Molecular Weight Distribution (MWD). To conduct rheological modelling and computations with Control Theory (CT) and Unified Models, it is necessary to utilize the Rheologically Effective Distribution (RED) related to MWD. The macrostructure of synthetic polymers is inherently heterogeneous. The term ‘polymer’ implies a composition of varying molecular sizes and structures. Unlike the classical normal or Gaussian distribution often encountered in the sciences, the representation of molecular weight distribution (MWD) on a logarithmic scale may seem somewhat unclear. In this section, we will clarify and compare these different distribution patterns, highlighting their unique characteristics and implications. Experience has shown that what appears to be straightforward regarding distribution has led to misunderstandings and incorrect conclusions.