The Ignition scripting API, which is available under the module name "system", is full of functions that are useful when designing projects in Ignition. From running database queries, manipulating components, to exporting data, scripting functions can help. Some of these functions only work in the Gateway scope, and other only work in the Client scope, while the rest will work in any scope.
"I'm upgrading from FactoryPMI - will my calls to fpmi.* still work?"
Yes. Ignition's scripting API is backwards compatible. You'll probably want to gradually move your "fpmi" references to "system" but you don't need to.