This appears to refer to Article 18(2) ff. of the original Stockholm Convention on POPs, which states that parties may submit a declaration stating that they recognizes one or both of the dispute settlement methods provided for:
http://chm.pops.int/Portals/0/Repository/convention_text/UNE...

In the original Austrian version of the convention, you can find that declaration at the very end after Annex F:
https://www.parlament.gv.at/PAKT/VHG/XXI/I/I_01171/fname_000...

However, in later versions that Declaration has been replace by an Annex G describing the dispute resolution process, so that is likely why they are referring to the Declaration made in the original statute.
https://www.ris.bka.gv.at/GeltendeFassung.wxe?Abfrage=Bundes...

Finally, one can see all of the declarations issued by the various countries in this UN document:
https://treaties.un.org/pages/ViewDetails.aspx?src=TREATY&mt...
Under the heading "Declarations" comes "Unless otherwise indicated, the declarations were made upon ratification, acceptance, approval or accession." And since Austria is listed as having ratified the convention, I'm suggesting "declaration made upon ratification" for "Erklaerung StF". (Otherwise, depending on the country, it might be "declaration made upon acceptance" etc.)