A light path expression (LPE) is a type of regular expression that describes a specific light path (or set of paths) based on the scattering events. The expression can be used to "select" the contributions of those kinds of paths to the output. You can use this to isolate very specific parts of the output when compositing, without having to write a custom shader or render that part as a separate pass.
Here are collected the most typical recipes for creating custom passes for rendering with Mantra, which cannot or were difficult to obtain with standard means.