You can find the Style Target choice field only in modules with a repeater-field Content type such as
- Team
- Gallery
- Image box
Style target option basically lets you choose whether to apply the Prebuilt Style (Border, Box, Card, Card Border, or Custom) to either the whole module or each repeater item.
First, you can find it under the Appearance group in the module.
When you choose Style Target = Module, the Prebuilt style or Custom you choose is applied to the whole module:
But when you choose Style Target = Repeater items, here is what happens: