<report-subject> allows to specify the subject of the document:

<overall-title>
specifies the overall environment the document belongs to. This could be the title of an overall project, an entire system, the overall title of a multipart book etc. If nothing of this matches, this element can be left out.
<main title>
specifies the major title of the document. This could be the title of a working group, the name of a subsystem, the title of a partial document etc.
<sub-title>
specifies the subtitle of the document. This could be the type of the document (e.g. user manual) etc.
<short-name>
specifies a short name under which the document is known in the project environment or the user community. This could be a mnemonic label.