How to define which resources(gifs,images,js) should be listed in EXTRARES

1. When I record a script by running a page, some URLs in EXTRARES are displayed in web_url.
2. Compared the URLs in EXTRARES with the the gifs, images and javascripts in “Resources” tab in Chrome Devtools, we can found that lots of URLs were shown in “Resources” tab but not in EXTRARES.
3. I think lack of those resources will cause less load generated to Web server.

Could you please let me know why those resources were not captured by LR ?