Each group, resource, and process type must have a Key and Label defined to uniquely differentiate instances of each type. While most of the time a unique field within the type is designated as the Key or Label, sometimes it is necessary to make a Key or Label that consists of more than one field. Each type gives the option of defining a Key and Label using EL. For defining a Label using EL you may write something like this: ${car_model} - ${car_engine} instead of choosing "Label" checkbox on any field.
Within the rule engine, you can use EL for getting access to any rule variables, from forms or device messages that trigger a rule, or any resource, group, or process found within the rule. Additionally, within a business rule the Set Variable action allows you to save the result of almost any function into a local or global variable. For example:
Define a local variable named "noaa" this way: ${nx:noaa_observation(s.s_location)}. The "noaa" variable is now an object, and you can get access to all its fields using EL. For example: ${_local.noaa.pressureIn}. Note that for getting access to a local or global variable, you have to write the prefix "_local" or "_global" (respectively) before the variable name. Similarly, when creating a query on the report builder, you can use EL to gain access to any field of the selected targets. Once you gain access to a variable using EL, you can then use any function or standard operator to operate upon that variable and create more complex values.