how idoc determine it is a surcharge or discount

we have a condition type ZF01 for freight charge and it is on header level. when the billing is created and transfer to an Idoc (RD04 output), it create two line under E1EDK05. one is E1EDK05 +/ZF01 which is correct. the other is E1EDK05 of ALCKZ - (surcharge)/KSCHL ZFRT. I have no idea how this E1EDK05 - from. we do not have a condition type of ZFRT.

but we have another condition type ZPF5 for freight charge. it works fine on idoc file. just create E1EDK05 + only.

I compare ZPF5 and ZF01, the only difference on condition type is condition category.

I am wondering how Idoc file know the condition type will be a surcharge or discount.

thank you~~