What does it do? Specifies the cardinality of the current CDS association:
If the attribute 1: is specified, the current CDS association is declared as unique.
If the attribute *: is specified, the current CDS association is declared as non-unique. The addition 1: makes it possible to use filter conditions in path expressions, which are used in WHERE clauses or HAVING clauses. In a clause like this, it overwrites the cardinality of the CDS association. (The variant of the path expression can, however, be affected differently on certain database systems.) The addition 1: or *: cannot be specified as the only addition in square brackets.
Latest notes: The addition 1: prevents a syntax error if a path specified with filter conditions or with a non-unique cardinality is used in a WHERE clause or HAVING clause. However, at runtime, there is no check to validate whether the condition achieves the required unique cardinality. NON_V5_HINTS ABAP_HINT_END
