PO files use the file format of the gettext tools. See http://www.gnu.org/software/gettext/
For information on KDE style PO files, see http://public.planetmirror.com/pub/kde/devel/gettext-kde/
# this is another comment
#. comment extracted from the source code
#: sourcefile.xxx:35
#, fuzzy
#_ note to translator (this is nonsense)
This style is no longer used by tools like pofilter, they use normal comments to ensure that they work with Gettext tools. Handling of these will be removed in teh future.
magid @_: within msgid\n@
Also know as KDE style comments as they are used by KDE for message disambiguation and comments to translators.
#~ msgid "Blah" #~ msgstr "Bleeh"
#| "Previous msgid"
In Gettext 0.16
msgid "English" msgid[af] "Engels"
Unsure of when or if this was added to Gettext.