The seed value is a starting point to generate masked values.
The Data Masking transformation creates a default seed value that is a random number between 1 and 1,000. You can enter a different seed value or apply a mapping parameter value. Apply the same seed value to a column to return the same masked data values in different source data. For example, if you have the same Cust_ID column in four tables, and you want all of them to output the same masked values, set all four columns to the same seed value.