A more thorough explanation:

Activity incident to service refers to an action taken by a member of the military as part of their duty or because of their status as a military member. This can include military operations or activities that are related to their service.

For example, if a military member uses a military aircraft to fly home, any injuries they sustain during the flight may not be eligible for compensation under the Feres doctrine. This is because the flight is considered an activity incident to service.

The Feres doctrine is a rule that prevents military members from suing the government for damages related to injuries sustained during military service. This rule was established in the case of Feres v. United States in 1950.

Another example of activity incident to service could be a military member participating in a training exercise or providing security for a military base. These activities are considered part of their duty as a member of the military.