You can limit which values appear when updating a Ref column by specifying a Valid_If expression on the column. For example, in an order capture application you might limit the allowed values to "open" orders or to orders totaling over $1000.
You expression must yield a list of key values for the referenced table. For example, if the referenced table is Orders and that table's key is OrderId, then the expression must yield valid OrderId key values.
Although the Valid_If expression must always yield valid key values, you can use Row Labels to display other values to the user.