At the crossroad of law and computer science, the notion of Digital Rights Management renews the research on Lex Electronica. Rights Expression Languages provide the legal semantic and syntax to be implemented by Digital Rights Management Systems. Rights Expressions are legal metadata that define the actions led on a digital text or multimedia file. This article presents some legal resources and requirements to express rights and transactions pertaining to digital documents and data, ie legally or contractually allowed usages or actions. The proposed model associate terminological resources from technology, usages, exclusive rights and public domain. Research around this article takes place in Medialex, an interdisciplinary project aiming at building a transaction ontology to license, access and reuse.