PSO44 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 "IS-PS-Specific Additional Fields" 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_PSO44 TYPE PSO44.
The PSO44 table consists of various fields, each holding specific information or linking keys about IS-PS-Specific Additional Fields data available in SAP. These include PSONF (Flag Additional Receivables), BGKEY (Line Item Category for Adjustment Documents), BGMWSKZ (Tax on sales/purchases code), BGKTOSL (Transaction Key)... 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. .