Codesys - SysSaveRetains??

Can someone explain how the SysSaveRetains block works? What type of file needs to be called at stFileName? Can't seem to find anything useful online or in the manual about this. Whatever filename I put, it gives me a -1 reading, which if I understand means that it could not locate the file

I'm using Codesys v2.3 with the wago 750-841

Thanks,
Martino

you have to open a file and the filename is retain.txt for example.
ask this on the 3s-software forum and you will get answer from rolf.