The Append tools combines each row of your first dataset with each row of your second dataset. It is typically used when you want to “attach” a couple of columns of static data to a larger dataset.
Be careful with the Append tool. The Append tool works by combining every row from each of the two input datasets. So if you have 10 rows in dataset one and 100 rows in dataset two, you will create an output of 1.000 (10 * 100) rows by appending them. This is also known as a cross join or a cartesian product and will slow down performance. If you have 10.000 rows and 10.000 rows, you will create an output of 100.000.000 - i.e. it quickly produces significant datasets.
Configuration
The Append tool doesn’t require any configuration. But you can add a custom suffix to any potential duplicate columns to avoid conflicts. 
