Rules and Rule Sets
A Rule is an if-then statement that says, if one or more conditions are met, then take one or more specified actions. A Rule Set is a collection of such rules.
About Rules, Rule Sets, and Rule Chaining
The ServiceDesk interface makes it easy to define rules by selecting fields and specifying matching criteria. When satisfied, the criteria causes any and all actions associated with the Rule to be carried out.
Rules can be defined in named Rule Sets, or they can be part of an unnamed set (a "Rule List") defined for for Event Based Rules, Timer Rules, and Scheduled Rules.
Each Rule can carry out one or more actions. One of those actions is to execute another Rule Set, which causes the Rules to chain. When Rules chain, the processing of the current Rule Set stops until the chained Rule Set finishes. The processing of the original Rule Set then resumes where it left off. (Chaining can go to arbitrary depth.)