Flow layout is the default block behavior where elements take 100% width of their container – essentially normal HTML block-level element behavior. This creates the foundation for WordPress’s flexible layout system.
Constrained layout enables content width restrictions through the “inner blocks use content width” setting. The key insight: combine constrained parents with flow children for maximum flexibility.
When a constrained parent changes content width, flow children automatically adapt without manual adjustment. Setting content width on both parent and child creates inflexibility and maintenance problems.
Flow blocks within constrained parents can still use alignment options (content width, wide width, full width) even though they’re in flow mode. This combination provides both structure and flexibility for responsive design.