Rule Sets Editor
From Runic Games Wiki
Rule Sets are built in the Rule Sets Editor. These determine what randomized chunks are used and how they fit together. The Level Rules also determine art-related things such as lighting, music, environment (floaty particles, rain, day-night cycle), aliasing (for example choosing between snowy or non-snowy areas), themes( mostly for turning on and off decorations in chunks so they can be reused for many locations ), and level features (used to determine what types of chests, special events and other things are used, and how many of them to use).
Example of the Level Rule Set Editor template
Level Rule Set Properties
To see the breakdown of each of the properties available in the Level Rule Set editor refer to the section below.