data: mem(7) type c value '12121'.
data: begin of gt_itab,
a(5) type c,
b(2) type c,
end of gt_itab.
field-symbols like gt_itab.
assign mem to casting.
Field symbols allow you to access data objects dynamically in ABAP programs. Field symbols are placeholders or symbolic names for other fields. They do not physically reserve space for a field, but point to its contents. A field symbol cam point to any data object. The data object to which a field symbol points is assigned to it after it has been declared in the program. Whenever you address a field symbol in a program, you are addressing the field that is assigned
so int the above code gt_itab is the data object. So this data object is assigned to FS.