400 feet is the FAA imposed ceiling (the highest you are allowed to fly by the FAA - in the US). Many hobby flyers argue that the 400 foot rule is a guideline, which hobby flyers can choose to obey or ignore. But 400 FEET is the current rule or guideline.
A "rule" is a US law and a "guideline" is a suggestion by the FAA. Here are the actual rules and guidelines from the FAA's website:400 feet is the FAA imposed ceiling (the highest you are allowed to fly by the FAA - in the US). Many hobby flyers argue that the 400 foot rule is a guideline, which hobby flyers can choose to obey or ignore. But 400 FEET is the current rule or guideline.