Shared Requirements are not programs, but instead are requirements that need to be added to programs in order to be visible. Therefore, their scope is much less meaningful - the scope that you choose for a shared requirement will not apply to those students - instead, the scope indicates which audits are eligible to use that particular requirement.
Unless you have a large number of shared requirements, or a large number of audit editors, there is no harm in indicating that all shared requirements can be used "university-wide" - this means that you'll never run into an issue where you're looking for a shared requirement that can't be used because that audit falls outside of its scope. If you want to maintain tighter organization of which requirements can be used in which audits, it is a good idea to apply a narrower scope to your shared requirements.