When you attach your test, you have the option just to show a part of the tree match or test results. I can't remember the wording for this. It was something like just show the part that matches or something. This isn't what you are referring to or is it?
In those trees where they have opted just to show a section, I wonder how it is determined which section. Does ancestry chose the section or does the person? I didn't chose that route so I don't know.