[Quoth]

I used to use "Annotations", but as I don't want them in Calibre, but in separate text files, the Kobo Utilities suit better. The format is easily changed in any good text editor after copy, paste, save. I used to use Notepad++ in Windows and now use KATE on Linux Mint.
You can then paste them back into Calibre.


Edit:
On Kobo you can also edit the config so there is an extra context menu item per ebook to allow export as a text file. Then you can copy it off using USB transfer in a file manager, but I found Annotations simpler, then Kobo Utilities suited my use case better.