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[ ] are used to define events of either/or. So in the example above I could say [RT] meaning one Reflection or Transmission type and [DS] which means Diffuse or Specular event. This is pretty common when you need to collect light along a path with many different possibilities. An example might be <[RT][DS]> which means a single event (note the angle brackets) with either a reflective or transmissive type (first set of square brackets) of a diffuse or specular event (the second set of square brackets).

Parenthesis

() are used to encapsulate a series of events. This operates most like the purpose of parenthesis in a mathematical operation, grouping items to be considered together. Such an example would be to have two different sets of events you want to collect but placing a pipe, | to say "or" between these encapsulated events. C(DS)|(TD)L would be Camera to Diffuse to Specular event or Camera to Diffuse Transmission event.

Defining Bounces

Since we're tracing through the scene collecting light, it might be important to specify how much light you need or want to collect. This is usually meant to separate the direct and indirect events mentioned above. But you may also specify how many bounces along this path you want to collect rather than let the renderer collect all of them.

To begin we'll look at the usual notation for Direct and then Indirect light collection. 

The Beauty Render

Let's look at an expanded beauty LPE. And by "expanded" I mean with all the parts spelled out and not in shorthand (which we'll cover later).

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