Data that could reasonably be expected to cause damage to national security includes classified information at various levels:

• Confidential data: Unauthorized disclosure could cause damage to national security. Examples include military strength, weapons information, and information that impairs programs or policies related to national security.
• Secret data: Disclosure could cause serious damage, such as disruption of foreign relations, revelation of significant military plans or intelligence operations, or compromise of scientific or technological developments related to national security.
• Top Secret data: Disclosure could cause exceptionally grave damage, including armed hostilities, vital disruption of foreign relations, compromise of complex cryptologic systems, or revelation of sensitive intelligence operations or vital national defense plans.

In summary, data types that might cause damage include diplomatic information, military plans, intelligence operations, scientific and technical developments, and any information whose unauthorized release would harm national defense, foreign relations, or critical security systems.