The head is one of the worst targets you can choose. Beside the brain, the tiny bones for hearing, the nose and the many small blood vessels can very easily be damaged. Especially the neck and the back of the head have very little padding. If you must hit in that area, slap the face with the flat hand. Do not use fists or any devices.

The upper arms can withstand more impact than head and face, however since they are so close to the head, anything other than a hand is probably a bad idea. Also, the inside of the arms have very little padding and any hard impact will immediately cause bruising. If you decide to use devices on the upper arms do so with care on the outside of the arm.

The spine is one of the worst places to hit. Especially thinner people where you can see the individual bones of the spine can suffer from severe damage when hit on the spine.
Unfortuantely it is not always possible to avoid the spine when working on someone’s back. In that case, try to hit the back as flat as possible, allowing the muscles and fat on either side of the spine to absorb most of the impact.

The back is one of the places that can sustain the most force. How much exactly is usually determined by how much muscle and fat a person has to avoid impact on the spine and shoulder blades. If you use inflexible toys, use them below the shoulder blades.

Small breasts do not offer enough protection for the ribs. That means that floggers and softer whips can be used in the area but paddles and other hard impact devices are a bad idea as they can easily bruise the rips. If you use floggers or whips be careful to make sure not to hit the face or neck.

Since large breast are mostly fat, they are fairly safe to hit. In addition to their sexual appeal, their tenderness makes them one of the favorite targets for beatings. Any device can be used on large breasts since they protect the ribs underneath very well.
There are many questions about if bruising of the breasts increase the risk of breast cancer. To my knowledge, no study in this area has ever been made but the general consensus in the BDSM community is that there is no danger.

Breast implants are designed to withstand a lot of abuse, i.e. the impact on an airbag. While the manufacturers do not release specific, it is considered safe to use anything but heavy paddles on breasts with implants. If you have saline-filled implants, the capsule is more likely to break before the implant. In most implants the capsule will reform over time and all signs of the capsulotomy should go away.

The forearms are fairly safe to hit. With the exception of elbows and wrists, all bones are fairly well covered and the blood vessels are large and not prone to breaking easily. Any whip or flogger can be used. Paddles are not recommended because of their impact can cause temporary loss of control over the hands.

The stomach has a layer of fat over some very strong muscles. This area is safe for anything but heavy paddles.

The lower back should be avoided with anything but the softest floggers because the spine is not covered in that area and the kidneys can also be damaged easily.

The hands contain many sensitive bones and tendons. The palms can withstand some mild torture. It is however difficult to hit the palm without impacting the fingers or wrist.

The hips are covered with nerves and blood vessels. Gentle flogging or whipping are alright Paddles generally are a bad idea.

The pubic area is another main area for sadists but because of the tenderness of the tissue, much care needs to be taken. Paddles are a bad idea, as are riding crops.

The butt is another favorite. It can withstand almost any form of punishment.

The inner thighs are very sensitive but with the muscles and fat, can withstand a great deal of impact.

The outer thighs are less sensitive than the inner parts but can also be hit very hard with any device. However, because of the lack of sensitivity and the resulting knowledge of when to stop, bruising is more common here.

Knees are absolutely off limits for any hard impact. A very soft flogger may be used gently but most people consider even that a bad idea.

The only part of the lower leg that is safe to hit is the calf. They can withstand even paddles but make sure to avoid ankles and knees.

Only the bottoms of the feet are safe to hit. The rest of the feet and ankles contain too many small bones and blood vessels. The bottoms of the feet can withstand a lot of impact but the force of a larger paddle can cause problems in the ankles.