Certain Nodes allow users to add Custom Properties by clicking the + button underneath the standard Properties.
From here, you can re-order, delete, add a note, rename and change direction of the property as well as adjust the Property data type.
The behaviour of the custom properties depends on the Node and custom properties are not supported on all Nodes.
Formatter - Custom properties provide a list of keys and associated values to replace out of a string expression.
Script - Custom properties cause C#-style properties to be defined in a partial class permitting C# script to interact directly with custom properties defined on the Node.
Variable Bar || All properties are custom. The Variable Bar provides a data contract (variables in and out) for the Workflow for use by an end-user or accessed via API.
XPath Match || Custom properties provide a list of keys to associate with values matched out of an XML document. These are output custom properties that can then be used elsewhere on the Workflow.