PDF/A is widely used for long-term preservation of documents. For example many governments (a.o. European, Austrian, Dutch, Brazilian, German, Swiss) recommend or mandate PDF/A. 

LibreOffice already supports exports to PDF/A-1, which is great. PDF/A-2 is the second part to the standard. It adds some significant extra features. Therefore it is important that LibreOffice will support PDF/A-2. 

In the below you will find some basic information on PDF/A-2 from Wikipedia and some pointers to more background information.
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PDF/A-2 address some of the new features added with versions 1.5, 1.6 and 1.7 of the PDF Reference. PDF/A-2 should be backwards compatible, i.e. all valid PDF/A-1 documents should also be compliant with PDF/A-2. However PDF/A-2 compliant files will not necessarily be PDF/A-1 compliant.

Part 2 of the PDF/A Standard is based on a more recent version, PDF 1.7 (ISO 32000-1), rather than PDF 1.4 and offers a number of new features:
* JPEG2000 image compression
* support for transparency effects and layers
* embedding of OpenType fonts
* provisions for digital signatures in accordance with the PDF Advanced Electronic Signatures – PAdES standard
* possibility to embed PDF/A files in PDF/A-2 for archiving of sets of documents as individual documents in a single file.[5]

Part 2 defines three conformance levels: PDF/A-2a, PDF/A-2b and a new conformance level PDF/A-2u. PDF/A-2u represents Level B conformance (PDF/A-2b) with the additional requirement that all text in the document have Unicode mapping.
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Background information:
- http://en.wikipedia.org/wiki/PDF/A#PDF.2FA-2
- http://www.iso.org/iso/iso_catalogue/catalogue_tc/catalogue_detail.htm?csnumber=50655
- http://www.digitalpreservation.gov/formats/fdd/fdd000319.shtml
- http://www.scl.org/site.aspx?i=xb965