This is a late response, but just incase someone else has this same question.
When you are using operations that first require a startReading or startEditing operation to be completed first, the sessionID must match. If performing these operations via the ArcGIS Pro client, the pro client will generate the sessionID and ensure that this same sessionID is used for all REST calls.
When you are using the REST endpoint yourself to issue these calls (through your application), it is your responsibility to generate your own sessionID. You can use an online GUID generator, python module, etc for this GUID. This is dependent on where you are executing these REST calls from.