SMLSD 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 "Supplemental fields (I/O table) for SMLS" 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_SMLSD TYPE SMLSD.
The SMLSD table consists of various fields, each holding specific information or linking keys about Supplemental fields (I/O table) for SMLS data available in SAP. These include MLST_MANDT (Client), AUTYP (Order category), MLST_SMLNR (Name of standard milestone), MLSET (Milestone group)... 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. .