Hi AnkitPal,

You are generating 16 digit unique number.

From that unique number to get the sequence number for automatically increment Substring is used.

The substring starts at a specified character position mentioned and continues to the end of the string.

For more details on Substring Method refer below link.

https://docs.microsoft.com/en-us/dotnet/api/system.string.substring?view=netframework-4.7.2