Allow null sets an attribute's nullability, i.e. if this attribute should be able to hold "no value". It affects both the in-memory value (it will show up with a question mark at the end of the type, like Boolean?).

In C# and OCL, it is a bit tricky to know to check for both null and '' (empty string) for strings. Hence, the ->isnullorempty operator on string is good to use.

Nullability is at its best for numbers - helping the user to distinguish if someone wrote 0 or no-one has written anything yet...

Read more about Initial values and Default Database values for a fuller explanation of the properties InitialValue, DefaultDBValue and Allow Null.