The Save to text file dialog box appears when you select the Save to text file... command (the File menu).  You use this dialog box to choose the output format and content of the output file.  After clicking OK on this dialog box, a file dialog appears for you to select the output file name.

Plain text with folder names
Select this to save the file as a plain text file without the .po formatting.  A long row of dashes is printed after each translation item.  If you select Original and translation, the original string and translation string are separated by a short row of dashes.  Folder names are printed before each group of translation lines in the folder.  Note that this is insufficient to represent the folder structure because it does not show folder nesting.  If you need this, use poTree format instead.

Plain text without folder names
Select this to save the file as a plain text file without the .po formatting and without folder names (i.e. just translation items).  A long row of dashes is printed after each item.  If you select Original and translation, the original string and translation string are separated by a short row of dashes.

poTree format
Select this to save the file in the normal poTree format, just as if you saved with the Save or Save as command.

Gettext format (remove poTree’s structural comments)
Select this to save the file in gettext .po format.  This removes the extra comments that poTree uses to maintain the tree structure.

Write BOM signature to mark the file as UTF-8 format
Selecting this option causes poTree to start the output file with the UTF-8 BOM signature (hex EFFFBF).  The output file is always written in UTF-8 format, but some text editors will not recognize the format without the BOM signature whereas other text editors may be unable to recognize the signature.  Therefore the option is provided for maximum flexibility.

Completed items (translated, not fuzzy, no errors)
Select this option to include completed items in the output file.

Untranslated items
Select this option to include untranslated items in the output file.  Note that this option only makes sense if you write the original strings to the output file.  Otherwise, the output file will be blank.

Fuzzy items
Select this option to include fuzzy items in the output file.

Items with errors
Select this option to include items marked as errors (but not fuzzy) in the output file.

Original and translation
Select this option to include original strings and translation strings in the output file.

Original only
Select this option to include only the original strings in the output file.  If you choose poTree or gettext format, the msgstrs will all be blank.  If you choose plain text, the translation lines will be omitted entirely.  This option can be useful for passing the original strings through an automatic translation program.

Translation only
Select this option to include only the translation strings in the output file.  If you choose poTree or gettext format, the msgids will all be blank.  If you choose plain text, the original lines will be omitted entirely.  This option can be useful for passing the translation strings through a spell checking program.