A rule specifies a set of procedural expressions that query model elements to :

A rule can query elements either in source and target models, and can create/update elements in source and target models as well. A rule is an atomic element of model transformation. A rule may call other rules to perform a complex task. Rules are grouped into logical units, called rulesets.

A rule may point to a parent rule, in which case it means :

Using subrules may help to organize a ruleset into logical rule trees. The main point is to share parameters: subrules don't have to redefine parent rules parameters.

Related reference
Ruleset structure
Rule statement