Since AN 2.1.5 is still a work in progress version. The node may not be final and documentation is missing.
Here is a working example. Hope it helps you.
You need to add and define the affected channel path.
This is not a node you can directly invoke with the SHIFT + A submenu. Here is how to do it :
Create your loop and click it to select it
Assign a parameter
Go to the right-side panel (shortcut N, or click on the arrow on the right side of the animation nodes node tree)
Go to the "Node" subpanel
Click on the "Reassign" button next to the parameter socket