In the last few decades, knowledge organization systems (KOS), especially thesauri, classification schemes and lists of subject headings, have largely followed or conformed with the established data models defined by standards, recommendations or best practices. This long list contains some widely used models, such as ISO5964 Part 1, ISO2788, Z39.19, BS 5723 and BS 6723, (Dextre Clarke, 2008) IFLA Principles Underlying Subject Heading Languages (SHLs), and MARC 21 Format for Classification Data. The FRSAD (Functional Requirements for Subject Authority Data) conceptual model is the third member of the FRBR family, developed under the auspices of IFLA. The report was approved in 2010 and will be published in 2011. FRSAD is a general conceptual model that focuses on the subject relationship and therefore provides a theoretical framework for all KOS and their data models. In addition, it also assists in the assessment of the potential for international sharing and (re)use of subject authority data both within the library sector and beyond. In this paper FRSAD is compared to SKOS and SKOS XL as data models (with implementation examples).