Hey Lily,
The difference between grouping and stacking:
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Grouping was created to facilitate the editing process of multiple elements at a time. Grouped elements move and behave together, so you can drag, resize, duplicate, arrange and align multiple elements more efficiently.
You should stack when you want to maintain a vertical distance between two elements (typically the relationship is with the parent). -
Stacking ensures that your elements stay in their vertical order on every screen size and in fact is a layouting tool, whereas grouping is an editing tool that lets you perform actions on multiple elements. Stacking allows you to create a relationship between elements, you can add vertical margins between two elements (typically the relationship is with the parent).
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Stacking elements that are already in a group will automatically ungroup them and create a transparent container so they never overlap.