After doing a little digging around, it turns out that these are the instructions that cataloguers have been given if they want to contribute RDA records to WorldCat:
Catalogers may contribute original cataloging using RDA to WorldCat if desired. (040 $e with value rda and Leader/18 (Desc) coded i if ISBD punctuation is used or blank if not).
So, there are a few points:
- The leader position is 18 not 17
- We are looking for either an "i" or a blank. If there is an "a". That would indicate AACR2 coding
- The 040 subfield "e" should be used in concert with leader coding to identify RDA records
So, it looks like we are making sure that the "a" is not present in position 18 in the leader and the subfield "e" is present in the 040.