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< > are used to choose/define a single event. For example, when deciding I want a specular reflection event, I can say <RS> for a reflective specular event. I cannot create an event that is either/or, meaning I can't say <RT> because that's two different kinds of events types (reflection and transmission). Instead it must be a single type and event and type. You can also put these in order to collect a light path in a specific order, such as <RS ><TD > as to say "one reflective specular event and then a transmissive diffuse event next".

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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

The Beauty Render

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