AFP IOCA Image Data shows the message "Unknown Self Defining Field" in IPD field

Hello,

I run the command below:
>fop -fo TEST.fo.modifFOP-2433.xml -afp TEST.fo.modifFOP-2433.xml.afp -c fopFOP-2433.xconf

If I modify the input file "TEST.fo.modifFOP-2433.xml" to use the bmp, gif, jpg, tif format images then parse the relevent afp files (attached) to analyze the afp structure. I found that in the IOCA Image Data section, there are many messages "Unknown Self Defining Field" in many IPD fields for those types of images.

But if I use the *.png images then there is no message "Unknown Self Defining Field" as well as only one IPD field.

For visualization, please see the attachment "images_in_AFP.doc"

I am newbie to FOP + AFP. Please let me know why there are differences above? and the messages "Unknown Self Defining Field" are errors?

Thank you!
Le Duy Nhac