What are the different types of service-connections?

Direct service connection: A disability is established for direct service connection if it is caused by or resulting from personal injury suffered or disease contracted in line of duty, in the active military, naval, or air service.

Secondary service connection: An additional disability, which is proximately due to, or the result of a service-connected disease or injury and not due to the natural progress of the nonservice-connected condition will be service connected.

Aggravation of preexisting nonservice-connected disabilities: A preexisting injury or disease will be considered to have been aggravated by active military, naval, or air service, where there is an increase in disability during such service, unless there is a specific finding that the increase in disability is due to the natural progress of the disease.

Aggravation of nonservice-connected disabilities: Any increase in severity of a nonservice-connected disease or injury that is proximately due to or the result of a service-connected disease or injury, and not due to the natural progress of the nonservice-connected disease, will be service connected.