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.
Your expression must yield a list of values from the key column of the referenced table. For example, if the referenced table is Orders and that table's key column is OrderId, then the expression must yield values that can be found in the OrderId column. For example,
SELECT(Orders[OrderId], OR([Status] = "open", [Total] > 1000))
A common mistake is to use an expression like
SELECT(Orders[Not the key column], ...)
with the intent of showing a different column than the key. The dropdown will still function and show the intended column, but selected values will not match the referenced key column and will result in broken references (indicated by an "!" icon). Instead, you should use Row Labels to control which columns are shown.