SMI_S_DM_ITEM_KEY is a standard SAP Structure so does not store data like a database table does. It can be used to define the fields of other actual tables or to process "Item key & object reference" Information within sap ABAP programs. This is done by declaring abap internal tables, work areas or database tables based on this Structure. These can then be used to store and process the required data appropriately. i.e. DATA: wa_SMI_S_DM_ITEM_KEY TYPE SMI_S_DM_ITEM_KEY.
The SMI_S_DM_ITEM_KEY table consists of various fields, each holding specific information or linking keys about Item key & object reference data available in SAP. These include VBELN (Sales Document), POSNR (Sales Document Item)... See below for full list along with technical details, documentation, text table, check tables, foreign key relationships, conversion routines, relevant tcodes and example ABAP select code etc. .