Check Table vs Value Table
It very important to know that a table behaves as both value table and check table but not simultaneously.
Consider a field F present in a database table DBT. Consider a Table T (Another database aTable) which behaves as a Value Table and Check Table at different Situations.
When the field F is a primary key for the database table DBT, the table T behaves as a Value Table. We can enter a new value for the field F and values for field F flow from table T the database table DBT. (Field F is the primary key for both the tables T and DBT)
When the field F is a foreign key in the database table DBT, the table T behaves as Check Table. We cannot enter a new value for the field F and Values for field F will be Checked against Table T and only those values present in table T should be given for field F. (Field F is the Primary key in Table T and Foreign Key in Table DBT).
Read Here for More SAP ABAP Interview Questions