In DataView a record is identified with a key called C_ID. This C_ID has a range from 1000000000 to 2000000000. A widget contains this field C_ID invisible and you can search via ECI in this field <Tablename.C_ID> .
Type widgets also contain the C_ID of the assigned entity record and relation widgets contain the C_ID of the assigned 'second entity' record.
Example:
Typelist EDB-DOC-DRW-TLI of document type DRAWING contains the two C_IDs T_DOC_DAT.C_ID and T_DOC_DRW.C_ID.