As per MacRumours, all iPhone 16 models with feature an Action Button that will be capacitive in nature. The ones on the iPhone 15 Pro and iPhone 15 Pro Max are mechanical buttons. This means the new Action Button could function like the Touch ID Home button on older iPhones or the Force Touch Trackpad on newer MacBooks.
The internal documents reveal the Action Button codename is Atlas and that it will have a force sensor capable of analyzing pressure and "tact-switching functionality". Also, the documents mention a Capture button on the iPhone 16 series that could function similarly to the Action Button.
The details are a little hazy but the company is expected to ship all future iPhones including the iPhone SE 4 with the Action Button. So, Apple seems to be ditching the signature Ring/Silent button in favour of this new button. MacRumours also claim Apple is working on bigger-sized Action Buttons too that look like the iPhone volume rocker.
In time, we should see more renders and leaks regarding this. For now, we can safely assume the Action Button to carry forward the following features: