\label{sec:implicitly}
One aim of RO-Crate is to be conceptually simple. This simplicity has been repeatedly checked and confirmed through an informal community review process. For instance, in the discussion on supporting ad-hoc vocabularies in RO-Crate, the community explored potential Linked Data solutions. The conventional wisdom in RDF best practices is to establish a vocabulary with a new IRI namespace, formalised using RDF Schema or OWL ontologies. However, this may seem an excessive learning curve for non-experts in semantic knowledge representation, and the RO-Crate community instead agreed on a dual lightweight approach: (i) Document how projects with their own Web-presence can make a pure HTML-based vocabulary, and (ii) provide a community-wide PID namespace under https://w3id.org/ro/terms that redirect to simple CSV files maintained in GitHub.
To further verify this idea of simplicity, we have formalised the RO-Crate definition (see Appendix \ref{sec:formaldefinition}). An important result of this exercise is that the underlying data structure of RO-Crate, although conceptually a graph, is represented as a depth-limited tree. This formalisation also emphasises the boundedness of the structure; namely, the fact that elements are specifically identified as being either semantically contained by the RO-Crate as Data Entities (hasPart) or mainly referenced (mentions) and typed as external to the Research Object as Contextual Entities. It is worth pointing out that this semantic containment can extend beyond the physical containment of files residing within the RO-Crate Root directory on a given storage system, as the RO-Crate data entities may include any data resource globally identifiable using IRIs.