Hi Long Ding, You are right, both the ‘reference set selector’ and the ‘input reference set selector’ can be used to select associated objects. Both only work where the association is a n-m relation. If this is a question in a course, you might want to send an email to mendix. They might fix it.
The difference between the two is the visual representation on screen, a list of values vs. a single line of concatenated values.