In order to support the Code | Surround With... action, the plugin needs to register one or more implementations of the SurroundDescriptor interface in the com.intellij.lang.surroundDescriptor extension point. Each of the surround descriptors defines a possible type of code fragment which can be surrounded - for example, one surround descriptor can handle surrounding expressions, and another can handle statements. Each surround descriptor, in turn, contains an array of Surrounder objects, defining specific templates which can be used for surrounding the selected code fragment (for example, Surround With if, Surround With for and so on).

When the Surround With... action is invoked, the IDE queries all surround descriptors for the language until it finds one that returns a non-empty array from its getElementsToSurround() method. Then it calls the Surrounder.isApplicable() method for each surrounder in that descriptor to check if the specific template is applicable in the current context. Once the user selects a specific surrounder from the popup menu, the Surrounder.surroundElements() method is used to execute the surround action.