The Stone Slab is a crafting material that can be made in a Processor.
2 Stone Slabs can be crafted from either:
1 Stone block,
1 Bedrock block, or
1 Limestone block
Stone Slabs can then be processed into 4 Stone Rods each if needed.
Stone Slabs are used for a mutitude of crafting recipes, like for (Basic) Extractors, Advanced Extractors, Forge, Obsidian Mining Cells, Stone Chests, Arctek Chests and other furniture, Stone Stairs and building blocks like Stone Floors and Roofs.
Like with everything crafted in the Processor there is no need to unlock any recipes for crafting Stone Slabs.