CMPB_ASGN_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 "Key for Campaign Assignment" 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_CMPB_ASGN_KEY TYPE CMPB_ASGN_KEY.
The CMPB_ASGN_KEY table consists of various fields, each holding specific information or linking keys about Key for Campaign Assignment data available in SAP. These include POSNR (Item number of the SD document), CAMPAIGN (Generic project planning: GUID from external R/3 system)... 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. .