Here is my test scene, one StaticPointLight and two metal static mesh, after build Lightmass, metal surface get very dark. Is baked direct light has little influence on metal material, or it loss quality in lightmaps, or other reflection rules in Lightmass?
When change PointLight to Stationary and build Lightmass, the metal get very bright. Does it means metal reflection is dynamic?
Here i add a Sphere Reflection Capture Actor and PointLight remain static, after build Lightmass, it looked not bad.If only use static light and want metal surface to have reflection effect, only add a Sphere Reflection Capture Actor?
As far as I know, metal objects look like metal because they are reflecting. If you have your static light and nothing else, the metal has nothing to reflect and therefore it appears dark. When your light is set to movable it is reflecting on your metal so it looks like metal. And with the reflectioncapture you have something to reflect on your static lit metal so it looks like it.
So yes, you are right you need the reflectioncaptures when you have a scene with static lights. 
Thank you, metal has nothing to reflect in static light, does it mean that direct light can not affect metal material while indirect light can in Lightmass? But metal diffuse is very small, so indirect light affect metal material very little and the metal looks dark. So reflectioncaptures is to help the metal have much more reflection?
puh. To be honest: I don’t know 
This was just an assumption. I think with a movable direct light it would also work. But I am definitly no expert in this topic.
