Table 2.

List of categories and their definitions

Category	Definition
Training	Challenges related to inquiries for more information on a specific tool, standard, or a general concept
Community	Challenges involving activities of peers in the same community to achieve reuse and prevent duplicated effort
Modelling	Challenges involving the conceptualisation of data into data elements and bindings of standardised vocabularies to these data elements
Implementation	Challenges involving implementation of a specific tool or standard
Legal	Challenges related to inquiries about data sharing and reuse agreements, informed consent, or implementation of related services

Five categories were created to organise the FAIRification challenges of RD patient registries. The categories reflect the nature of the challenges: the need for training, to learn from others, information about modelling, implementation, or legal aspects