Block pickers determine the block to use to place as a block state in the world. The block picker type is determined by the type field, and all types are specified below. You may also inline it by only specifying the block, in which case minecraft:simple is used.

Always places the block specified.

"minecraft:cobblestone"

{
  "type": "minecraft:simple",
  "block": "minecraft:cobblestone"
}

Places the attached block when used on the attached side, otherwise uses the other block. It is typically used for blocks that have a separate block when on a wall.

{
  "type": "minecraft:attached_to_side",
  "attached_block": "minecraft:torch",
  "attached_side": "down",
  "other_block": "minecraft:wall_torch"
}