The Custom ID is an alternate identifier that is controlled to be unique, each document can have a Custom ID.
Here you define how the uploaded documents will be renamed. The rule must determine the name of the document.
Independently from the folder where the document is originally uploaded, you might want to automatically move it to the right folder accordingly to your own logic. In this case the rule must return a path that will be used as the final destination of the document, if this doesn't exist it will automatically created. If the path starts with a / character, it will interpreted as an absolute path otherwise it will be considered as relative inside the original upload folder.
In this panel you can see the list of the sequences currently active into the system.