Bug 11317 introduced in Koha 18.05 a new variable "access_dir" which is nowhere documented in the Koha manual, as far as I could see. In new Koha installations the following can be found in "koha-conf.xml"
but it is nowhere explained how to set or how to use these veriables.
In older versions of Koha that are updated these variables are missing and as far as I can remember there is no notice that they should/could be added.
However, if you click Koha menu "Tools > Access files" you will get the message:
Error: Files could not be found because the "access_dir" option was not set in "koha-conf.xml". Contact your system administrator to add this option.
There is section https://koha-community.org/manual/20.11/en/html/tools.html#access-files in the manual but it just says "The directories where the files are stored need to be defined in the koha-conf.xml file." without further explanation.
The access_dir is used as the path or paths for the "Access file" tool, there is no other use I am aware of.