Hi Katharina,
It sounds like you're looking for a definition of the tem "referenced requirements".
When you import requirements that were authored in a third-party tool to Requirements Toolbox™, you can choose to continue to manage the requirements in the third-party tool by importing the requirements as referenced requirements.
Importing requirements as referenced requirements allows you to continue use the third-party tool as the source of truth for your requirements while gaining access to Requirements Toolbox analyses like implementation status, verification status, and change tracking.