Which command initializes a program to stop until the next command?

The command that initializes a program to stop until the next command is M0. This command is known as a "program stop" or "optional stop" command. When M0 is encountered in the G-code program, the CNC machine will halt its operation until the operator intervenes. This allows for manual checks, tooling changes, or other necessary adjustments without the machine automatically resuming operation.

In the context of CNC programming, using M0 can be particularly important during operations that require verification or safety checks, ensuring that the operator has control over the machine's actions before proceeding. This command is critical for ensuring the safety and precision of the machining process.

The other options, while related to CNC programming, serve different functions: M1 is an optional stop that can be activated based on conditions, M2 signifies the end of a program and resets the machine, and M3 typically starts the spindle in a clockwise direction. Each of these commands has its specific usage, but none provide the unconditional halt and pending command resumption that M0 does.