Expansion vs aws_secrets syntax

While using terraspace with it’s awesome templating feature I have a question regarding the syntax of the expansion method vs the aws_secret helper:

"<%= expansion('csdap-cumulus-:ENV-:INSTANCE-tfstate') %>"
vs
"<%= aws_secret("csdap-cumulus-#{Terraspace.env}-#{@mod.options.instance}-urs-client-id") %>"

Is there some place on the documentation where the available attributes of the Terraspace object on the aws_secret helper are shown? Would this be the correct way to reference the instance name?

Thank you!