It would be nice if your Atom package might call some code in another Atom package, right? To do this, one package can provide a service and another package can consume that service.
Package One provides a service -- a function that can log the string "A Thing!" to the console -- and Package Two consumes this service, which newly enables Package Two to call the string-logging function in Package One.
package1.js contains a function called ProvidePackage1() that returns a function or an object, and Package One's package.json file registers ProvidePackage1() as a provided service in a providedServices field. (The versions subfield is so you don't break services as you update your package: you can introduce new functions for new versions while still providing the older versions' services.)
package2.js contains a function called ConsumePackage1() that receives and binds the provided function or object. Package Two's package.json registers this function in a consumedServices field. (You have a versions subfield here, too, to consume a specific version of a provided service.)
Fire up Atom with the repo's two packages loaded -- call apm link package1 package2 from your local repo clone to load these packages on Atom startup -- and open Atom. ctrl+alt+l will log a string from package1, while ctrl+alt+L will use package one's provided service to log the same string via package two. option+cmd+i will open your developer console and show you the logged strings, as well as which lines in the codebase logged them.