E621 is a Chemical Flavour Enhancer.
Both Link and Reload were originally Mark Pritchard solo projects/aliases. Then, Tom Middleton joined Pritchard on some of these releases and "& E621" was added to specify his involvement or "flavour enhancement" of the project. (ie: Reload & E621 / Link & E621) With Middleton's heavy involvement on the Reload "A Collection of Short Stories" album (with no specific credit as "E621"), Reload came to mean both Pritchard and Middleton, and the "E621" alias was scrapped.
No tracks were ever released by "E621" alone.