In the linked ACLU post, the 2nd-5th bullets are a great example. The second bullet specifically enumerates the government position on border searches. The 4th and 5th bullets detail what the Border Patrol can do away from the immediate vicinity of the border.
But the 3rd bullet, which is key to the notion of the 100 mile "constitution free zone", is more mysterious. It states that Customs has "broad—though not limitless—powers" without detailing them.
Many readers of this ACLU post and coverage of it have come away with a false idea that Customs and Border Protection has some magic wand. The facts are very different.
Here are the referenced sections, forgive any formatting issues.
> - According to the government, however, these basic constitutional principles do not apply fully at our borders. For example, at border crossings (also called "ports of entry"), federal authorities do not need a warrant or even suspicion of wrongdoing to justify conducting what courts have called a "routine search," such as searching luggage or a vehicle.
> -Even in places far removed from the border, deep into the interior of the country, immigration officials enjoy broad—though not limitless—powers. Specifically, federal regulations give U.S. Customs and Border Protection (CBP) authority to operate within 100 miles of any U.S. "external boundary."
> -In this 100-mile zone, Border Patrol agents have certain extra-Constitutional powers. For instance, Border Patrol can operate immigration checkpoints.
> - Border Patrol, nevertheless, cannot pull anyone over without "reasonable suspicion" of an immigration violation or crime (reasonable suspicion is more than just a "hunch"). Similarly, Border Patrol cannot search vehicles in the 100-mile zone without a warrant or "probable cause" (a reasonable belief, based on the circumstances, that an immigration violation or crime has likely occurred).