Here is an attempt to answer that question. I am not sure my answer is completely correct though, so I hope someone with better insights can correct me if it is wrong, including terminology.
@ - base project/environment
@v#.# - default project/environment (e.g. v0.7 in this part of the docs)
@stdlib - standard library project/environment
This is what makes up the stacked environment available in the REPL, allowing one to load code from base, stdlib and packages the user adds to the default project.