SLS Specified Cubemap?

I am having a lot of difficulty trying to simply get Unreal to recognize any .hdr or .exr files under the SLS Specified Cubemap selection in the Skylight.

My first question
What exactly is SLS Specified Cubemap?
I assumed it is a traditional 6 image cubemap in either horizontal or cross formation but that doesn’t seem to be the case.

2nd question
How do you convert any of the HDR images online so that they can be recognized by Unreal when using this setting?

The cubemap has to be a spherical cubemap, like the ones you can find here:

Unrreal Engine usually only recognizes hdr as cubemaps, so if you have an exr, you would have to convert it into an hdr. If you download one of those from hdrihaven, UE should recognize it as a valid cubemap, that you then also can use as an SLS Specified cubemap (no clue, what SLS specified actually means ^.^ ).

Oh, if your imported hdri turns out to be pixelated, make sure, that you set it´s max ingame resolution as high as possible. It can happen, that it´s downscaled to 1024 pixels, or lower, for whatever reason.