Which of the following is NOT a function of spatial reference in ArcGIS Online?

In the context of spatial reference in ArcGIS Online, the primary functions revolve around defining how geographic data is represented. Spatial reference encompasses a set of parameters that define the coordinate system, map projections, and accuracy of representations.

Defining the coordinate system is crucial, as it determines how the locations in the dataset relate to the real world. This includes specifying whether the data is projected or geographic. Similarly, setting map projections is integral to ensuring that the data is represented accurately in two dimensions, as projections help depict the Earth's curved surface in a flat format. Ensuring representations are accurate is also a vital function, as it guarantees that the spatial information aligns correctly with other data layers, maintaining spatial integrity.

On the other hand, controlling access to data is not a function of spatial reference. Access control typically pertains to the management of user permissions and security settings for datasets and maps, rather than how the data itself is geometrically represented in a spatial environment. Thus, the correct answer highlights a distinction between spatial reference functions and data governance features within ArcGIS Online.