What is field symbol?

A field symbol does not physically reserve space for a field, but points to a field
which is not known until runtime of the program. Sometimes you only know which fieldyou want to process, and how you want to process it, at runtime. For this purpose, you can create field symbols in your program. At runtime, you can assign real fields to such field symbols. All operations which you have programmed with the field symbol are then carried out with the assigned field. After successful assignment, there is no difference in ABAP/4 whether you reference the field symbol or the field itself.