Transaction numbers allow you to group entries with the same document number and date, so that you can balance them together. Transaction numbers usually begin with the number 2 each year. The number 1 is reserved for the opening transaction, which is automatically created each year. The only exception is for the first accounting period in the first year. In this case, the opening transaction is the number 1.
When you enter a transaction number in the first line of a journal, the same transaction number is automatically entered into the other lines if the entries are not balanced. If the entries are balanced, the next number in the sequence is automatically entered into the other lines.
Having a unique sequential transaction number, ordered by date, allows you to identify every transaction posted in a fiscal year.