With this window you can edit the theater's strings (or the Default strings). The usage is pretty straightforward, but the important thing is what is written in the red frame on the window.
All strings in tac files are stored as a number, which references the actual string in this file. For Nowhere the number is 0, North Seoul is 4 etc. If you delete a string entry and hit Save this will save the corresponding idx and wch files and the numbering of the strings will change (i.e. the new number for North Seoul might be 3 now).
Since in the loaded tac file, North Seoul is still stored as 4, the program will change all string entries to the new numbers, in order for all the names not to be changed (if an old entry doesn't have a new one it will be 0). The tac file (unlike the idx and wch files) is NOT saved automatically. You need to hit Save for the tac in order for these changes to be saved.
That means, that if you change the numbering of the strings and hit Save and for any reason you don't save the tac, next time you open the tac file, the names will be wrong. Moreover, any other tac file with objectives using the same strings will have wrong names.
The solution here is either don't delete any strings or use the following trick.