Adding the package to its workspaces

I am looking into the workspaces feature, but have trouble understanding what is the correct way to add the package to its workspace einvironment: dev or add. For example, in the Pkg.jl manual, it is suggested to add the package to the test environment using ] dev .. On the other hand, Documenter.jl guide suggests adding the package as ] add PkgName to the docs environment. Since the package may not be even registered yet, using dev seems like the more robust way to add it to workspaces. But ...