When used in a referred block of text, such as a quotation,
mark indicates that the highlights were not made at the time of original authoring and is are being looked at, being considered for editing, for example.
When used in the general run of a document,
mark indicates that the highlights are likely relevant to the user’s activity. This could be highlighting terms matching a search, for example.
<blockquote> <p>He wants to play with his <mark>Legos</mark></p> </blockquote> <p>The person being quoted is clearly American because, for some odd reason, they pluralise "Lego".</p>