The description of the format entry.
The number of characters that should be there in the substring. It should be an integer and greater than zero. Note: Numbers greater than Number.MAX_SAFE_INTEGER will result in rounding issues.
The starting position in the original string from where the substring should be extracted. It can be either a positive or a negative integer. If It's negative, the counting starts from the end of the string. Note: Numbers greater than Number.MAX_SAFE_INTEGER will result in rounding issues.
The Substring masking format extracts a portion of the original column value and uses it to replace the original value. It internally uses the Oracle SUBSTR function. It takes the start position and length as input, extracts substring from the original value using SUBSTR, and uses the substring to replace the original value. To learn more, check Substring in the Data Safe documentation.