All children that refer to the parent will be deleted.
Note
If the reference does not identify the relation this can
result in the automatic deletion of many records. For example if the relation
between the parent and child is not one-to-one but one-to-many.
Restricted (with counter)
Deletion of the parent is restricted if any child refers to the
parent. A reference counter is checked, which indicates how many children refer
to the parent. With each database action this counter is checked and updated,
if necessary.
Nullifies
The field(s) of the child, which refer(s) to the parent that
has been deleted, will be made empty (spaces or zero value).
No check
If the parent has been deleted, no check or action will be done
on the child occurrences which refer to the parent. In that case the
referential integrity of the database cannot be guaranteed.
Lookup (Restricted)
Deletion of the parent is restricted if any child refers to the
parent.