The placeholder blocks (also called substitution blocks) are tan and brown. The tan block is the placeholder block, and the brown is the solid placeholder block. Each serves its own function.
Using the Placeholder Blocks
The placeholder block tells the mod to leave any blocks that are currently in that spot alone. It will not remove or change anything that already exists in that spot.
The solid placeholder block tells the mod to replace any non-solid blocks (air, grass, leaves, etc.) with the biome’s regular ground block (grass, sandstone, red sandstone, white terracotta, etc).
You can make and use these blocks in survival, but they are mainly used to scan schematics.
Here’s an example: