Compress image, compress JPG, reduce image size in KB: the slider is for a look you choose, and the target box is for a limit a form gave you, such as 100 KB. The search keeps the highest quality that fits, then scales the picture down only if quality alone cannot get there.
The quality slider re-encodes a JPG or WEBP at the percent you pick, and writes a PNG again when the file is a PNG. PNG has no JPEG-style quality step in the browser, so that slider does not change a PNG. Target-size mode binary-searches quality, then shrinks the long edge if even the lowest quality is still too big. A PNG that cannot land under the target is saved as a JPEG, and the note says so. Strip EXIF is on by default. Re-encoding already drops metadata; unchecking the box copies a JPEG EXIF segment back onto a new JPEG when one was present. It cannot put JPEG EXIF into a PNG.
A quality setting is a preference. A target is a ceiling. The two controls are separate so a 100 KB limit is not confused with “make it look a bit softer”.
Before and after sizes are the real byte counts, and the previews are the pictures the browser produced. A batch downloads as a ZIP. This is not compress PDF. A PDF redraws pages and makes text unselectable. A photo should stay on this page. The file converter changes format without chasing a kilobyte target. A target is a best effort: if the picture cannot get that small without becoming a speck, you get the smallest attempt and a sentence that says the target was missed. The page does not label a 400 KB file as 100 KB.