This working paper has been prepared by members of the SSHOC, EOSC Nordic and FAIRsFAIR projects working on related topics. It seeks to present some key concepts and expand on them to specify the standards and assessments required for an interoperable ecosystem of FAIR (findable, accessible, interoperable and reusable) data preserved for the long term in generalist and specialist FAIR-enabling trustworthy digital repositories (TDR) for a defined designated community of users. It seeks to provide context and define these concepts for audiences familiar with research data and technical data management systems, but with less direct experience of digital preservation and trustworthy digital repositories (TDR). This is intended to progress a conversation about how we define and specify requirements for the different types of services that play vital roles in the digital ecosystem. For repositories offering long-term preservation the relevant definitions and expectations are relatively mature and can be validated through TDR certification such as the CoreTrustSeal. The intention is also to help clarify which organisations are potential candidates to receive CoreTrustSeal TDR status and to identify and support the types of organisations that may not be candidates but also play a vital role in the data ecosystem.