All about Elevation
The elevation is only used on our cards and floating items such as floating buttons. If you put an elevation on a card, it symbolizes that the card is clickable. If the card is not clickable, there should not be used an elevation on it.
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Image and flag card
If you choose a simple card there are theses kinds of elevation: Normal, special and flat. These kinds have a different meaning. A normal card is a clickable card with a lower elevation. Special is also a clickable card with a higher elevation if you need an eye-catching card. Flat cards are for non-clickable cards.
If you choose e.g. a header card, the default elevation is set to z2. We have an elevation scale z0, z2, z4, z8. You can change the elevation depending on the purpose. If the card should be clickable then change it to z0. If you want a noticeable card because it different from the other elements on the page, then change it to z4. However don't use z8, it's only for floating elements.
Image and flag cards only have an elevation at z2. You cannot change it, because they are supposed to be used as a clickable card.