From "3 The Format of PO Files" at
http://www.gnu.org/s/hello/manual/gettext/PO-Files.html ::
Comment lines starting with #: contain references to the program's
source code.
The reference comment is useful to jump to the source position.
GNU gettext tools support the reference comment. e.g.: po-mode.el binds
"s" key to "po-cycle-source-reference" that opens a source position in
a new buffer.