After using the Polygonal Lasso tool to circle, press CTRL+J. The result is just a copy of the selected layer, without cutting the original image. Please help me explain what is going on. Is it where I set it? .

Ctrl+shift+J didn't cut out the sky. Why?-PHP Chinese Network Q&A

After using the polygonal lasso tool to circle, pressing CTRL J results in just copying a layer of the selected area without cutting the original image. Please help me explain what is going on. Is it me? Where is it set?