what is two domain level concept in abap????????????

Table fields are defined in SAP R/3 by data elements and
domains. This is a key difference between SAP R/3 and many
other systems.
 In other systems, a field is defined only by its type
(i.e., character, numeric) and length. These characteristics
are assigned directly to each field.

In SAP R/3, a field is defined by being assigned to a data
element, which is in turn assigned to a domain. These
assignments determine the field’s type and length in
addition to other characteristics.

SAP R/3 refers to this method of field definition as the
Two-Level Domain Concept