As you understand, Sub Participants who are not using STP do not receive the BO5 sub-item patern about STP.
The "Change Reason code 43" is not set for the case of Sub Participants who are not using STP.
For examples:
BO5 sub-item patern of order deletion by non-STP reason.
34901+34005+34904+34902 Change Reason code "1"
BO5 sub-item patern of order deletion by STP reason.
34901 + 34005 + 34904 + 34902 Change reason code "43"