This concludes the list of mandatory features and the distinction between traditional and object-oriented database systems should be clear. Relational database systems do not satisfy rules 1 through 8. CODASYL database systems partially satisfy rules 1 and 2. Some people have argued that object-oriented database systems are nothing more than CODASYL systems. It should be noted that (i) CODASYL systems do not completely satisfy these two rules (the object constructors are not orthogonal and object identity is not treated uniformly since relationships are restricted to be 1:n), and (ii) they do not satisfy rules 3, 5, 6, 8 and 13.
There is a collection of features for which the authors have not reached consensus on whether they should be required or optional. These features are: