ELI stands for "extra low interstitial". Essentially, this is a controlled chemistry version of the standard 6-4 product with lower oxygen content. To qualify as a true ELI product, it should be produced as such at the mill. Some suppliers, however, have been known to check the chemical test report and "re-certify" the product themselves. Be sure to ask your supplier if the material you are buying has been produced and certified properly.