The "Subject source" is set to short or long text. First the explanation of the FSM field length for the "Subject" and the "Notes". Below shows the current Service Call DTO v21 for subject and notes max length.
This first line in long text is filled as the subject of the FSM service call.
As can be seen in "/PACG/ECM_TESTTRANSO - Test Output Transformation" below for subject source = "Short text"
For long text, the below transformation is seen.
Notes in FSM always refers to the second line of the long text of SAP ERP order. However, in this case, second line of the long text is also shown in subject above but will be truncated once it reaches the 512 byte limit.
Please also note that the idoc display (like tcode: WE05) will not show the entire length of the text as it's max segment length is 132, hence the need to use the output transformation tool to view the entries.