When you configure key or random masking for string data type, configure source filter characters to choose the characters that you want to mask.
When you set a character as a source filter character, the character is masked every time it occurs in the source data. The position of the characters in the source string does not matter, and you can configure any number of characters. If you do not configure source filter characters, the masking replaces all the source characters in the column.
The source filter characters are case-sensitive. The Data Masking transformation does not always return unique data if the number of source string characters is fewer than the number of result string characters.