Transactions is the unit of work by important principles are that the unit of work will succeed only if all the commands that are in the unit of work, it must all succeed.
Transactions functional unit is an important principle at that. The unit's work is accomplished only when all commands are in the works to be done at all.
Transactions is a functional unit by the principle that The unit of work will be done when every command in the functional unit that had to be done at all.