When dissecting the Object field of the comment (which contains the actual verbiage of the comment and other information) the first value is TYPE. This type determines the type of the comment that is stored.
The comment types for those added to an AP Invoice are as follows:
|Invoice Note/Report/Check Comments||Type=A|
|Invoice Report Comments||Type=D|
|Invoice Check Comments||Type=C|
Keep in mind that to have the comments print on a check, not only would you need to have an Invoice Note/Report/Check Comments or Invoice Check Comments the users need to make sure that when the payment processing jobs (AP155/AP160 or AP161) needs to be set to Print or Include Comments.