Annotation descriptions from Wikipedia:

Annotation is a summary made of information in a book, document, online record, video, software code or other information, "in the margin", or perhaps just underlined or highlighted passages. Annotated bibliographies, give descriptions about how each source is useful to an author in constructing a paper or argument. Creating these comments, usually a few sentences long, establishes a summary for and expresses the relevance of each source prior to writing.

The above quotation is a description of the annotation, and in fact it's not much different from the annotations on an XPS document.

The XPS annotations itself can include many types of content such as typed text, handwritten notes, and also highlight color.

Users are given three choices: highlight annotations, text annotations and ink annotations.

• Highlight annotations: this feature allows user to highlighting lines of text or paragraphs to draw attention to items of interest.
• Text annotations and ink annotations: user can adding text notes or ink notes to flag information for later reference.