machines/systems/flexibot/docs/marlin/_gcode/M400.md

tag, title, brief, author, group, codes, notes, parameters, example

tag	title	brief	author	group	codes	notes	parameters	example
m0400	Finish Moves	Wait for all moves to finish	thinkyhead	motion	M400			pre code Wait for moves to finish before playing a beep M400 M300 S440 P100

This command causes G-code processing to pause and wait in a loop until all moves in the planner are completed.