This is the way the trails and ribbons were coded in unreal so this is what index 0 or 1 does in that particular case but ultimately the index could mean many different results.
If you work with your own meshes you can create multiple UV sets and the index is going to pick which set a particular texture is going to refer to.
I’ve used spheres with 2 UV sets in the past. One with a planar projection, the other for spherical. I can’t remember why exactly, but I needed some textures to behave differently within a single material so I would pick index 0 or 1.
To give you another example, our characters have 2 UV sets. One were the character artists do whatever they need, the other one which we use for effects.
In their set, hands, eyes, body are split on different materials.
On the second set, the whole body is there and it’s layed out so the scale is even. This set is used for localized ingame blood splatters and global effects such as petrifying a character at run time.