C1DEFLTS parameter E2ELOGMBR value is stored in Endevor control blocks during initialization by program BC1PINIT.
This happens at the start of the ISPF Endevor or QuickEdit dialog, at the start of any batch utility like C1BM3000, or at the start of an started task spawned at the request of Endevor Concurrent Action Processing, Endevor workbench or Endevor Web Services.
Note that, in the case of Web Services, the use of the STC pooling feature has an effect on this since Endevor initialization happens at the start of the STC and, with STC pooling, the initiation and termination of started tasks is not triggered by the receipt of incoming Web Services requests.
The member itself is read and parsed at the end of the first element or package action performed during the Endevor run, right before the point where any defined user exit 03 would be called or, for package actions, after any defined "after package action" user exits would be called. If the member parses without errors, then E2E logging is performed for the action as specified in the member.
The E2ELOGMBR member is not parsed again during the Endevor run. Subsequent element or package actions in the same run are logged using the definitions parsed for the first action.
Therefore, in order to enable changes to the end-to-end definitions, foreground users need to get out of Endevor or Quickedit, batch jobs need to terminate, and web services users need to wait for new started tasks to be spawned.
This is how our programs are written as of today (July 23, 2018). This logic might change over time.