Each submission has its unique id which by default is a sequential number but you can change it to have a custom format.
1.- Go to settings/Entry Numbering
There are three settings you can change
|Number of Digits||The number of digits that the id is going to do. For example if the next number is 1 and you configure it to have 5 digits the next number will be 00001|
|Prefix||A text to add before the number, this option support tags.|
|Suffix||A text to add after the number, this option support tags.|
Changing the next id number
The count start with 1 but you can define which number is going to be the next by clicking change and define the next number