You have defined a mapping and a mapping step.
Cross-client extensions are carried out after SAP standard mappings, that is, you can use them to redetermine target fields that have already been filled by way of SAP standard transformations. Extensions are assigned to an owner, who defines the call-up sequence. The following owners are possible:
1 SAP Industry extension
4 Partner extension
6 Customer extension
The SMT sets an owner automatically if possible. If the extension is in a customer system, for example, then the owner Customer Extension is set automatically. Extensions are carried out in the following sequence: SAP industry extension - partner extension - customer extension. Customer extensions can, therefore, overwrite target fields that have been set with partners and SAP industry extensions. Checks on whether the value of a target field is exceeded are not active for extensions.