Outer and Outmost

Outer adjective - Situated on the outside or farther out.

Outmost is a synonym for outer in external topic. In some cases you can use "Outmost" instead an adjective "Outer", when it comes to topics like exterior.

Outmost adjective - Situated at the farthest possible point from a center.

Outer is a synonym for outmost in exterior topic. You can use "Outer" instead an adjective "Outmost", if it concerns topics such as external.