These are schemas that are directly and indirectly referenced, as well as schemas that are linked to any of the directly or indirectly referenced schemas in any way. For example, if we extend our previous example so that Schema Z includes Schema B, then Schema Z is linked to Schema A (because Schema A includes Schema B). Further, if Schema Z, which is linked to Schema A, is either the target or source schema of an .mfd file, then that .mfd file is considered to be linked to Schema A.
The file Web Service.wsdl is also linked to Schema A because it uses Schema Z which includes Schema B and Schema A includes Schema B.
The file Stylesheet.xslt is linked to Schema A because it is referenced by instance.xml, which in turn references Schema A.