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.