What does selecting "Set to Frame Size" do to a clip?

Selecting "Set to Frame Size" adjusts the clip to fit the sequence's frame size while maintaining the original video's aspect ratio and integrity. When this option is chosen, the clip is scaled down or up to fit the timeline without cropping or losing any part of the footage. This means that the visual quality and essential details of the clip are preserved, allowing the content to be viewed as intended, regardless of how it fits into the overall project size. It ensures a smoother workflow, especially when working with various clip resolutions and sequence settings, making it easier to achieve a professional look without compromising the video quality.