How to delete Material slots? Meaning of Slot names?

Development Asset Creation
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Hello,
I have spend today some hour with UE4.26 crashes at export to Substance Painter (LiveLink).
Before textures have been overwritten so I gave texture slots other names - this lead to crashes.
What is the meaning of texture slots? Is it not possible to define them free?

Then I couldn’t delete unnecessary material slots and imported in May, remeshed, deleted all UV sets except one.
AND: when I reimported in UE4 as a new mesh with new name, there again was the predefined material slot (only one) and again the old material was assign

So I wonder how to set them to default and how to get a clean workflow, when using existing assets with existing material slots to texture them new in substance.

Thanks