Miscellaneous section hold settings which don't feet anywhere. Currently are two:
Here you can put your own character which serve as marker for keyboard accelerator indicator in GUI. By default it is & (ampersand), but in some programing toolkits it may vary. For example in Gnome/GTK translations the underscore character "_" is the marker for the keyboard accelerator.
For unexperienced user "regular expression" sound strange. However, you're advised to change default value only you know what you are doing. Some GUI programming toolkits provide own context information description methods. Consult an experienced developer if you translate PO files other than KDE specifics. For the sake of completness I "translate" for you what default regular expression mean: "the text match if it start with _: and is followed by one or more characters and end with a newline".