Give me more information on requirement 3.1.21 in the NIST SP 800-171 revision 2 document.
Limits on the use of organization-controlled portable storage devices in external systems include complete prohibition of the use of such devices or restrictions on how the devices may be used and under what conditions the devices may be used. Note that while external" typically refers to outside of the organization's direct supervision and authority  that is not always the case. Regarding the protection of CUI across an organization  the organization may have systems that process CUI and others that do not. Among the systems that process CUI there are likely access restrictions for CUI that apply between systems. Therefore from the perspective of a given system  other systems within the organization may be considered "external" to that system."
Limits on the use of organization-controlled portable storage devices in external systems include complete prohibition of the use of such devices or restrictions on how the devices may be used and under what conditions the devices may be used. Note that while external" typically refers to outside of the organization's direct supervision and authority  that is not always the case. Regarding the protection of CUI across an organization  the organization may have systems that process CUI and others that do not. Among the systems that process CUI there are likely access restrictions for CUI that apply between systems. Therefore from the perspective of a given system  other systems within the organization may be considered "external" to that system."