Parsoid enriches the HTML representation of the wikitext it converts with various mechanisms, depending on the end goal of the information.

Data can be represented:

• as named attributes of HTML tags (or even as tags themselves),
• as part of the data-mw attribute,
• as part of the data-parsoid attribute.

HTML attributes

HTML attributes should be used for information that is required to read the page. "*Read*" should be interpreted broadly enough to encapsulate various uses by client-side javascript, such as giving different styling to template-generated content.

Examples of these attributes would be the typeof and the about attributes used for template identification.

data-mw

The data-mw stores data that is required to edit content: elements that, in particular, VisualEditor may need to enable the editing of the page. For instance, the parameters of a template are stored in data-mw so that VisualEditor lets users edit them.

The data-mw data is typically semantic information that can be useful for all clients, including editing ones.

data-parsoid

data-parsoid stores Parsoid-private data. It is primarily syntactic information required for clean round-trips between wikitext and HTML. It may for instance contain information about template parameter names or whitespace.

VisualEditor never sees data-parsoid; most clients do not see it either unless they require it explicitly. Information there can be changed without notice.

The information stored in data-parsoid is documented on https://www.mediawiki.org/wiki/Parsoid/Internals/data-parsoid.