Is the below screenshot what you are talking about? Probably not since you said it’s an on/off settings. The screenshot is for iCloud. Other email services may be a bit different.
The difference is that if you save the Drafts in a local iPad folder, then your draft email can only be accessed on that iPad.
If you want to be able to continue to edit draft emails on other devices (iPhone, computer, etc) then you should set the Draft folder to the server, and use the same folder for all devices. That way you can start an email on the iPad, save it to drafts, and then complete and/or sent it from your iPhone or computer later.