The database executes the row-level security policy function you assign to the constraint object that corresponds to a table column to enforce the security policy for a data row. You can create a scalar function for each of the four request types (DELETE, INSERT, SELECT, UPDATE) that can be executed against the table on which the security constraint is defined.
See Teradata Vantage™ - SQL External Routine Programming, B035-1147 for information about how to write row-level security policy UDFs.
See Teradata Vantage™ - Advanced SQL Engine Security Administration, B035-1100 for general information about row-level security.