KPU3_CALLBACK 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 "Callback Structure for Plan->Plan, Actual->Plan" 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_KPU3_CALLBACK TYPE KPU3_CALLBACK.
The KPU3_CALLBACK table consists of various fields, each holding specific information or linking keys about Callback Structure for Plan->Plan, Actual->Plan data available in SAP. These include CB_PROGNAME (CALL program), CB_FORMNAME (Name of the Subroutine), CB_INFO50 (Callback: Character field 50 for application)... 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. .