Hello Tobias,
maybe it is strange, but it works this way. If you create your cancellation request the status of your customs declaration the Status will be set to 4A Cancellation Requested in the table /SAPSLL/CUHDSTA. So, the System "knows" what to do. The additional text is needed because the custom want to knows the reason for you cancellation request.
You can also check the FM /SAPSLL/CHECK_DE_EXP_DAT_21 - Incompleteness: Check Modules for DE. Here you find the Perform txt_0002_can_chk using is_ecc_cuhd-hd-d_txt changing lv_consistent.
If you have time and good nerves, you can even debug this process. Sorry that my explanation does not explain everything. I hope your confusion has become a bit smaller.
Regards,
Thomas