|DITA authoring concepts|
In DITA, format has a different meaning to style. Likewise, the meanings of data and metadata are very different. These distinctions in meaning are important to understanding the broader concept of the separation of content and form.
The word "style" in "Style Guide" is problematic, because it has a number of subtly different meanings in this context. Style could mean aesthetic presentational style, and it could mean writing or wording style. A style could be clean and crisp (aesthetically) while being ponderous and wordy (stylistically).
To distinguish between the two "styles", format (or presentational style) should be used when referring to aesthetical style, or the look and feel of the deliverable document. The term writing style should be used when referring to the authorial style.
In the broader concept of the separation of content and form, writing style belongs to content, while format belongs to form.
The DITA Style Guide touches on writing style, but does not address format at all. It focusses on semantic mark-up, something that is not usually necessary in conventional style guides.
It is also important to understand the distinction between data and metadata. Data is analogous to content, while metadata refers to information about the content.
For example, the data of a topic is the information that the reader reads about the subject matter of the topic. The metadata is the supporting information about the topic that the reader doesn't normally read or see, such as the creation date, the author, the semantics of the textual components, and the copyright ownership. Metadata is usually more important to the author than it is to the reader.
Both metadata and data are in topics, but ditamaps should only contain metadata. (It is technically possible to store some data in ditamaps, but you should avoid this practice.)