Our Catalogue is continuosly updated; we add the stamps sets as soon as we find data for scheduling. Then we do not follow a specific timeline. For almost all of the countries we decided to insert first all stamps sets issued after 1900, because their classification and data retrieval are easier.
Definitives.org use its own Catalogue numbers for stamps and set of stamps in the Catalogue. For example: ita1po07.
In the page with datasheet of stamps, beside watermark name you can see two orange icons: first one () let you see a preview image of the watermark; the second one () brings you to the page with watermark datasheet.
Although we give an availability indication for each stamp (common, not common, rare), stamps quotation is out of our purposes: we always recommend to refer to the quotations of the most known stamps Catalogues.
From CATALOGUE in the main menu at the top of the page choose the continent and reach the list of the countries actually in our Catalogue. Otherwise you can use the combo box at the bottom of this page.