LibreOffice » xmloff
Contains ODF import and export filter logic.
The main library "xo" contains the basic ODF import/export filter implementation for most applications. The document is accessed via its UNO API, which has the advantage that the same import/export code can be used for text in all applications (from/to Writer/EditEngine). The filter consumes/produces via SAX UNO API interface (implemented in "sax"). Various bits of the ODF filters are also implemented in applications, for example sw/source/filter/xml.
There is a central list of all element or attribute names in xmloff/inc/xmloff/token.hxx. The main class of the import filter is SvXMLImport, and of the export filter SvXMLExport.
The Import filter maintains a stack of contexts for each element being read. There are many classes specific to particular elements, derived from SvXMLImportContext.
Note that for export several different versions of ODF are supported, with the default being the latest ODF version with "extensions", which means it may contain elements and attributes that are only in drafts of the specification or are not yet submitted for specification. Documents produced in the other (non-extended) ODF modes are supposed to be strictly conforming to the respective specification, i.e., only markup defined by the ODF specification is allowed.
There is another library "xof" built from the source/transform directory, which is the filter for the OpenOffice.org XML format. This legacy format is a predecessor of ODF and was the default in OpenOffice.org 1.x versions, which did not support ODF. This filter works as a SAX transformation from/to ODF, i.e., when importing a document the transform library reads the SAX events from the file and generates SAX events that are then consumed by the ODF import filter.
There is some stuff in the "dtd" directory which is most likely related to the OpenOffice.org XML format but is possibly outdated and obsolete.