What is the difference between elementinstancekey and Jobkey
Jobs and tasks are different concepts in Zeebe. A job represents the work that needs to be done by a job worker to complete a certain task. A task is the element that is modeled in the process itself. When the task is activated in a process instance, a job is created for the task. Next to tasks, there are also other elements in a process (for example gateways), therefore we describe specific instances of these elements as element instances. So an activated task in a process instance is an element instance. There should be a job belonging to that element instance. Keys are simply the unique identifiers to these objects.
With that knowledge I hope elementInstanceKey and jobKey make more sense.