*The following descriptions are not universally (applying to all degrees of severity or even applying to healthy people and athletes) valid. Parts of the findings have not been academically secured to a sufficient extent but have been successfully proven in practice for several years. It is the attempt to assist the estimation with examples. Only if exertion is estimated "correctly" the Borgscale qualifies to control e.g. a physical training. Only then, the body can be kept in a certain area of exertion for an intended period of time and thus a defined adjustment can be triggered. Those zones are not fixed boundaries but should be considered fluent passage.
* Regarding your breathlessness: Not only try to ask yourself: "How severe is the breathlessness"? but also try to consider the aspect "How exhausting is breathing for me?". Breathing heavier with increasing exertion is normal as well as an increased perception of exertion. The point at which very strenuous breathing becomes a breathlessness differs individually. On some days (e.g. considerable weather changes) or with a progressed degree of severity one might experience breathlessness even at rest - without perceiving breathing as very being strenous.
The Borgscale is used to describe how exhausting a current activity or a training unit is actually (perceived exertion). In the training for people with respiratory diseases, one distinguishes the "degree of physical exertion" and the "degree of breathlessness". The perception of the physical strain (exertion) varies individually.
Apart from environmental factors (cold, mist, heat) the observation is influenced by breathing work, fear and exhausted muscles. For an estimation, numbers from 0-10 are assigned to the current feeling, 0 being the best and 10 the worst state. The descriptions are assigned to the numbers so that for instance if 4 means "somewhat hard", 2 ("light") should be half of this intensity-experienced as half as exhausting.
Using the Borgscale to supervise and control training is only reasonable if the perceived exertion is stated as spontaneously and honestly as possible. A comparison to others, who e.g. walk as fast, is not possible. Also, an estimation of the same physical load (e.g. 50 watts on a cycle ergometer) can vary on different days.
Example: Borgscale estimation for a flight of stairs which is too steep
In order to get a clearer picture of the numbers described try this: walk up a long and steep flight of stairs at a pace which you cannot keep up for very long. Depending on your state, you would find yourself in the range between 0 and 2 at the beginning. Within a short period of time (after approx. 15-45 seconds), you would reach a range of between 4 and 6. Shortly after, the physical load would exceed your current exercise capacity: you would continue to struggle for a few seconds (numbers 7-8) and then stop (termination of exercise: number 9 and 10).