“Test projects are expected to experiment common instruments, policies, strategies and action plans already developed to validate concrete solutions to be transferred.” What can be considered as ‘common instruments, policies, strategies and action plans already developed’. Are they only papers or documents one? Or could an already developed methodology, tool, or technology be considered as an ‘instrument’ and therefore be appropriate if we were to test it in a different setting or environment?

Common instrument refers to methodologies, tools, technologies, services, partnership/cooperation agreements developed in common, in addition to policies, strategies or action plan. It could indeed be an already developed solution that is tested in a different setting.
Test project aim at validating solutions or policies, strategies and action plans while transfer project instead aim at transfering already validated solutions, policies, strategies, action plans to other stakeholders for them to implement or adopt them, or to mainstreaming them into their own policies.