Halogen-Free Flux Vs Halogen Flux

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Hello,
I am using the SMD291 (Resin flux) and the NC551-10CC flux. I found the SMD491 flux which is ROL0 (Rosin) flux like the NC551-10CC, but the SMD491 flux is Halogen-Free Flux. Do you know what that means in practice and if the below that I read on the google is right. Also, could anyone expalin (or tell me where I can read about it) me what is/means the "smaller profile process window". Also, as I saw the SMD491 flux and the NC551-10CC flux have the same characteristics except from Halogen.


https://www.google.com/search?q=why+halogen-free+flux&sca_esv=6d0155d54e7c5b71&sxsrf=ADLYWIKlKGhzVGRsjBJJRxDNi8OMzfSluQ%3A1732921741473&ei=jUlKZ5HGHL6Rxc8P9_PUyQc&ved=0ahUKEwjRwMvG1IKKAxW-SPEDHfc5NXkQ4dUDCA8&uact=5&oq=why+halogen-free+flux&gs_lp=Egxnd3Mtd2l6LXNlcnAiFXdoeSBoYWxvZ2VuLWZyZWUgZmx1eEiLElCXC1jzD3ABeAGQAQCYAX6gAcsDqgEDMC40uAEDyAEA-AEBmAIEoALpAsICChAAGLADGNYEGEfCAggQABgTGAcYHsICCRAAGIAEGBMYDcICCBAAGBMYDRgewgIMEAAYExgFGAoYDRgewgIIEAAYgAQYogTCAgUQABjvBcICBhAAGAcYHpgDAIgGAZAGCJIHAzEuM6AHoQw&sclient=gws-wiz-serp