Image based reflections like what the topic title says. The whole lighting and material system depends on it. The reflection capture actors are just that: they capture an image (a cube map) that is then used as an image based reflection source and the engine blends them together with SSR.
i know that the reflection spheres are image based reflection but as far as i know that feature was Dynamic and the reflection are not static here is the documentation !
Dynamic components - all parts of the reflection except the static shadowing can be changed at runtime
so i think that not all features are in UE4