FMUP_INV_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 "Extension for FM Update: FI Key of Invoice" 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_FMUP_INV_KEY TYPE FMUP_INV_KEY.
The FMUP_INV_KEY table consists of various fields, each holding specific information or linking keys about Extension for FM Update: FI Key of Invoice data available in SAP. These include KNGJAHR (Fiscal year for FI document number), KNBELNR (FI document number), KNBUZEI (Document item for FI document number)... 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. .