The class Polygon_offset_builder_traits_2<Kernel> provides a model for the
PolygonOffsetBuilderTraits_2 concept which is the traits class
required by the Polygon_offset_builder_2 algorithm class. The class
Polygon_offset_builder_traits_2<Kernel> has one template argument: a 2D CGALKernel. This parameter must be a model for the Kernel concept, such as the Exact_predicates_inexact_constructions_kernel, which is the recommended one.
It is unspecified which subset of the kernel is used in this traits class (and by extension in the builder class). This is to avoid restricting the choices in the implementation.