Technically, mixed content refers to any element containing character data. However, in common usage it refers only to the latter case. The term is inherited from SGML and comes from the fact that the text in the XML document following the element tag is parsed looking for more markup tags. ANY means that the element can contain zero or more child elements of any declared type, as well as character data. It is therefore a shorthand for mixed content containing all declared elements.
Elements can have zero or more attributes.
Attributes can be optional, required, or have a fixed value. Attribute a uses a simple enumeration. Attribute a uses an enumeration of notation types. See the XML specification for complete details. These attributes point from one element to another.