Hi. I'm also using UDF features in my software with UG/Open API. I create my UDFs with the user interface and then export them as whole parts using UF_MODL_export_udf. After that I insert these UDFs in another part with UF_MODL_import_udf. If you do the same... I haven't found any way to explode this type of UDF. But I think, that in the UG API another method of work with UDF exists. Using user interface you can create UDF that consist of selected features (not the whole part). But it's really a good question who to do it using API. If you know how to do this, pleese, share with me. Thank you