I think an inline list component and a list relation component need ‘Multiple’ to be selected to work. A Relation component needs ‘Multiple’ to not be selected for it to work.
For a relation column, I tend to use Multiple in most cases since I don’t use the Relation component very often. I see it making more of a difference when using a Relation column in combination with a Lookup column. If you intend to use a Lookup with a Basic text component, then you cannot select Multiple on the Relation column. If you need an array of multiple values for an image carousal or per user data with multiple emails, then multiple will need to be selected for the relation column.