Philological text editions that are ‘born digital’ can and should be linked to other primary sources and, ideally, this happens within an ecosystem of clear and short IRIs. Yet, linking a text to other texts has always been a common practice in critical editing: by putting notes into the source apparatus, the editor constructs parallels between the text being edited and other primary texts. This paper explores how to formalize the knowledge contained in source apparatuses of existing editions. Based on the analysis of existing proposals and on the experiences of the OTRA project (Ontology for Re-Use and Argumentative Patterns), I propose five descriptive dimensions that allow modelling the knowledge that is already contained in the source apparatus. Setting up this infrastructure based on Linked Open Data would allow distributing its creation and federating its hosting among different actors such as libraries, archives, and museums.