A life jacket marked "100N" represents a life jacket with a buoyancy of 100 Newtons. Newton (N) is the unit used to express force, and in this case it refers to the buoyancy of the life jacket. Here are some key points regarding lifejackets and the designation of 100N:

1. Buoyancy (Newton): The buoyancy of a life jacket is measured in Newtons (N). In this case, a life jacket with 100N provides a buoyancy of 100 Newtons.

2. Purpose: A 100N life jacket is generally suitable for protection during shorter exposure to rough water, such as when sailing in coastal areas. It is intended for use in sheltered waters where rapid assistance is available.

3. Buoyancy aids: Lifejackets with 100N buoyancy are often considered buoyancy aids and not full-force lifejackets (e.g. 150N or 275N). They are suitable for swimmers near shore or for use in situations where help is quickly available.

4. Design: These lifejackets are generally designed to hold the wearer in a vertical position with the head above water. They often have a collar that supports the head and keeps the wearer's face out of the water.

It is important to consider the intended use, specific water conditions and the wearer's skills when choosing a life jacket. For situations with more challenging conditions or for individuals who cannot swim well, a life jacket with higher buoyancy may be more appropriate.