One of the key uses for a custom transformer is to repeat a pattern of transformers throughout a workspace. Reusing a custom transformer can be done the same way as with a normal FME transformer, in one of these ways:
Locate it within the transformer gallery and place a new instance.
Select the custom transformer, right-click to display the command menu, and choose Duplicate.
Copy and paste within the same workspace. If two compound transformers have the same name, Workbench will add a _2 suffix to the copy.
Copy and paste from another workspace. The definition is embedded into the current workspace.
You cannot use a custom transformers within itself, but you can use one custom transformer inside a different one.
When a custom transformer is to be used in multiple locations (for example, after you duplicate it), it is important to understand how each instance of that transformer references other parts of a workspace as input. See Working with Multiple Copies of Custom Transformers.