Structured annotations (or SAs), introduced in the 2008-09 schema, are a general way to extend OME-XML with additional structured information. They can express a variety of data types and linkages, and serve as a replacement for the Custom Attributes and Semantic Type Definitions functionality of prior schemas.
For more information on SAs from an OMERO-centric perspective, see the OMERO page on structured annotations.
Map Annotations, storing ‘key-value pairs’, are a type of Structured Annotation which were introduced in the Changes for January 2015. Further information is available in the OMERO developer documentation on Key-value pairs and a sample OME-XML file is also available.
The structure of the SA used in the schema is shown below, along with all the possible attachment points in the model.