The main purpose of this legislation is to change the rules around the detention of individuals who are in the United States but have not been formally admitted. It aims to establish a requirement for mandatory detention for these individuals, ensuring that they are held while their immigration status is being determined. Key provisions of the legislation include amendments to existing laws that clarify the process for inspecting and detaining certain individuals. It specifies that anyone who is physically present in the U.S. and seeking admission will be subject to mandatory detention.