the subject matter; the matter mentioned in the heading/subject line
A set phrase used in formal writing, especially in business emails and documents, to refer back to the topic stated in the subject line or heading. Often used at the beginning of a message to indicate that the following content is about that subject.
I am writing to you regarding the matter mentioned in the subject line.
Regarding the subject matter, please check the attached file.
Compound of 掲題 (けいだい, 'subject heading') + の (possessive particle) + 件 (けん, 'matter'). 掲題 is itself a compound of 掲 (けい, 'to put up, to post') and 題 (だい, 'title, topic').