How is the directional change of cast iron soil fittings referred to?

The directional change of cast iron soil fittings is referred to in fractions of a circle because this method provides a clear understanding of how much the fitting alters the flow direction compared to a full circle. This terminology allows installers to conceptualize and communicate the fitting's angle of turn relative to a 360-degree circle, making it easier to determine the proper fitting to use in a plumbing system where changes in direction are necessary.

The distinction of fractions of a circle is particularly useful in plumbing as it relates directly to the physical arrangement and flow characteristics of piping systems, ensuring that the angles are accurate and facilitate proper drainage and venting. By using this terminology, professionals can align their work with the precise requirements of building codes and standards. The other options do not provide the same clarity or common industry understanding for describing directional changes in fittings.