FMMDSUB_STRUC 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 "Fields of pop-up to enter new substring during MD creation" 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_FMMDSUB_STRUC TYPE FMMDSUB_STRUC.
The FMMDSUB_STRUC table consists of various fields, each holding specific information or linking keys about Fields of pop-up to enter new substring during MD creation data available in SAP. These include MASDAT (Account Assignment Element), STR_ID (Master Data Subdivision ID), SUBNUM (Not More Closely Defined Area, Possibly Used for Patchlevels), SUBVAL (Substring 1 of Commitment 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. .