What does when the scope of a topic has a certain focus mean?
It means that the writer should learn to write better. "The
scope of a topic" is the range the topic covers. "Birds" is a topic
with a greater scope than "vultures" which still has a greater
scope than "The feeding habits of vultures". Topics which have a
limited scope might be said to be more focussed. Thus it is wrong
to say that the scope of the topic has a focus. It is the topic
itself which has focus. The author has written something like "the
height of the man is tall" rather than "the man is tall".
It is not clear what the words "a certain" mean here. Unless the
author intends to identify the focus of the topic, they have an
effect opposite to that they appear to have. "That girl had a
certain something about her" means that whatever the quality of the
girl was it is clearly uncertain.
If the writer goes on to name the focus, then what he or she
intended to write was "when the topic is focussed on . . ." then
naming the focus of the topic. If not, the phrase means something
like "when they are talking about you-know-what"