A Cap block is a block that is designed to stop a script or project from running, cancelling unwanted activity before it can start.
As Cap blocks are designed to stop a script or project, their bottom is smooth; no blocks can be placed under them, as there is no need. Other than that, their shape is identical to the Stack block — they still have the notch on the top, so the block can be placed below others.
The need to stop a script (or all scripts) can be found in several occasions. Some include: