Border

Border blocks are unbreakable blocks that prohibit movement through, over or under them. They are exclusive to the Education Edition.

Usage
When placed, all blocks above and below the border block will be filled with invisible blocks that cannot be broken, and no blocks can be placed on top and bottom of the border block. When you break a border block, all invisible borders on top and bottom of the block will break and then you will be able to place blocks.

Trivia

 * Border block and cobblestone wall have same block models and inventory models.

Reference
Bordure Border 边界