Imagine we have an iframe (#iframeFoo) and need to query a sub-element within its scope. So we create a target for the iframe container:
Defining iframe container target
Then we create a target for the local element
Defining local target
The first one is a simple target like we are used to have in Puppetry, but the second is chained one. While building it we expand Options, set Parent target to IFRAME_FOO and Parent type to inline frame.
As soon as the local element target defined (IFRAME_FOO_LOCAL) we can address it as any simple target.