SAP Enterprise service are based on GDT / CDT and GDT/CDT can be used in modelling of Custom Services , this GDT / CDT has to be mapped to the corresponding backend field(s) always in the same manner.
Global Data Types/Core Data Types
XSD schemas are generated in the Enterprise Service Repository (ESR) per GDT according to their properties defined in the ESR (XI Repository).The XSD schemas are used on the ABAP side to generate ABAP data types (and proxy parameters) within the transaction SPROXY. Where the XSD data type based message content is transformed into an ABAP-based content before the mapping to the backend data types take place and vice versa during outbound mapping.
We need understanding how the XSD data types are mapped to ABAP data types and common ABAP code to assist in mapping during inbound (request) and outbound (response) mapping.
Attached link provides detailed information how ABAP data types are mapped to XSD data Types
SAP standard class “CL_GDT_CONVERSION” with static methods can be used to map from GDT to ABAP (inbound) and ABAP to GDT (Outbound).
ABAP GUID Representation is : “32113455645777777777777777777777”
XSD/GDT Representatio is: “32113455-6457-7777-7777-777777777777”