Determined in code by those lines (for jpeg):

• image_transport_plugins/compressed_image_transport/src/compressed_publisher.cpp

  Line 84 in f0afd12

  compressed.format = message.encoding;

• image_transport_plugins/compressed_image_transport/src/compressed_publisher.cpp

  Line 116 in f0afd12

  compressed.format += "; jpeg compressed ";

• image_transport_plugins/compressed_image_transport/src/compressed_publisher.cpp

  Line 127 in f0afd12

  compressed.format += targetFormat;

And those are the commits where format field changed:

• e9c2768
• 8a029c4

First added new form (original color format + ";jpeg compressed").
Second added also internal (intermediate) color format (original color format + ";jpeg compressed " + intermediate color format

The changes date back to 2012 so it is probably documentation that is not up to date.

The documentation should probably say something like:

ORIGINAL_FORMAT + "; " + CODEC + " compressed " + COMPRESSED_FORMAT
Valid codecs are: jpeg, png
example:
"bgr8; jpeg compressed bgr8"

But it gets more complex due to depth encoding...

.. depth encoding reuses the same message

and here compression_format is like png, rvl

So for depth it is like:

ORIGINAL_FORMAT + "; compressedDepth " + CODEC
Valid codecs are: png, rvl
example:
"16UC1; compressedDepth png"

Looking into the code empty CODEC is interpreted as png for backwards compatibility

Fix provided in ros/common_msgs#184 . Feel free to leave some comments there. And thanks for the research you did!