Identity and Access Management (IAM) is a crucial aspect of online interactions, and researchers and practitioners have been exploring this area since the early days of the Internet. Over the years, many digital identity protocols and standards were created. While OAuth 2.0 and OpenID Connect (OIDC) are currently the most widely used and mature protocols, the emerging concept of Self-Sovereign Identity (SSI) and its underlying protocols promise greater user privacy and control. Although much emphasis has been placed on new protocols and implementations, the ontological and formal understanding of digital identity models still needs to be addressed. In this paper, we make three contributions: (i) introduce the Role-Artifact-Function (RAF) framework, which comprises a meta-metamodel and a metamodel; (ii) describe and compare digital identity models through the instantiation of the RAF metamodel; and (iii) present significant insights, including the conceptual equivalence between the established family of x509-based protocols and SSI, as well as the correlation between the chronological evolution of identity models and their increasing linguistic complexity.