The papers which have already been accepted are published online with the label Accepted paper online. Such papers are published before the full number is closed. The reason for such policy is to make the accepted publications accessible to the readers as soon as possible. Such accepted papers are selected for quick online publication for which not only the scientific but also the editorial side is adequately advanced.
The accepted papers are already ascribed to the volumes and numbers, and the page numbering is known, so for a paper with this label the full bibliographic data are already fixed and they are given alongside with the title and author(s) information. Therefore, the paper available in this mode can be easily cited. The only difference between the Accepted paper online and the regular paper is that the number it appears in is not completed yet.
At present, the Accepted papers online can be found in Vol 21 (2012) No. 1/4, Vol 22 (2013) No. 1/4, Vol 24 (2015) No. 1/4, Vol 25 (2016) No. 1/4, Vol 26 (2017) No. 1/4 and Vol 27 (2018) No. 1/4. The presence of accepted papers online in the past issues is closely related to the journal's recovery process after discontinuities during the change of the publisher.
The most recent issue (containing the Accepted papers online) is Vol 27 (2018), No. 1/4.
The most recent closed issue is Vol 23 (2014), No. 3/4.