D1.8
5.1.3 Backing
The Supplemental Welder Qualification for Restricted Access Welding test shall use the type of backing to be used on the project, except that tests performed with no backing shall qualify the welder to use steel backing in production, but the reverse shall not be permitted.
Following what Scott quoted is text for "If backing other than steel is used, or no backing..." and then the Commentary for this section says "open root" as part of separate testing as all various backing must be separately qualified. In other words, if you take the test with steel, that is the only one you can use, not copper or ceramic. And, if you take it with ceramic, you can't use steel. Thus, if you take it open root, that is what you are qualified for.
Not quite like D1.1. In this case, you can't take the open root and be qualified to weld with backing. Backing IS an ESSENTIAL variable. That includes 'weld as backing'.