Most drivers today are concerned about the boiling temperature of their brake fluid, and not too concerned if it meets DOT specifications or not.
This is ok for drivers who only use their cars at the race circuit. Ordinary drivers who frequently drive their cars on ordinary streets should be more concerned if their fluid meets DOT specification. Brake fluid that do not meet DOT specification can speed up the deterioration of brake components over a extended period time. It can also lead to the malfunctioning of ABS during cold weather. Most people do not know much about brake fluid, and it can be hard to find relevant information to learn more about it.
Please read the following section, 'Understanding Brake Fluids' is an easy to understand guide explaining brake fluid.

DOT is the abbreviation for the 'Department of Transportation', which is an American government transportation department. The DOT set standards such as FMVSS (Federal Motor Vehicle Safety Standard) very similar to the JIS in Japan or DIN in Germany. The following table shows the DOT brake fluid specifications:

• □ Dry Boiling Temp. : Boiling point when the fluid is brand new, no moisture absorption.
• □ Wet Boiling Temp. : Boiling point with fluid that has 3.7% water by volume. After 1-2 years of fluid use.
• □ Viscosity : a measure to represent the brake fluid flow property. Higher the value, the more difficult for the fluid to flow. If the value is high when the air temperature is low, 　the fluid can have a negative effect on ABS performance.
• □ pH value : value to show acidity / basicity of a solution. If the pH value is lower than 7.0 (strong acidity), the fluid can accelerate corrosion of other brake components

The temperature at which the fluid boils. Water boils at 100℃、where as brake fluid with high boiling point will boil around 300℃, and low boiling point brake fluid will boil around 140℃.

When driving aggressively on windy mountain roads or race circuits, brake pad can reach over 300℃. This high heat gets passed onto the brake fluid through the calipers, which can raise the fluid temperature over 200℃. If the brake fluid is repeatedly heated past it's boiling point, some of the fluid vaporizes and creates bubbles within the brake lines. This is a very dangerous situation since this can lead what is commonly know as vapour lock, or simply the brakes not working. This occurs since the vapour is compressed instead of the fluid so the brake pads do not move.

The most common ingredient of brake fluid is glycol-ether. This fluid is hygroscopic which mean it absorbs moisture from the atmosphere. The 'percent by volume' is a measure of the water content in the brake fluid.

This is not exactly correct. The DOT numbers categorizes the fluids by various uses.

DOT 5.1 has strict viscosity standards at lower temperature in addition to having a high boiling point temperature. Therefore, in cold climate areas, the DOT 5.1 brake fluid is very commonly used on most cars. The most widely distributed brake fluid is the DOT 4, which has a dry boiling point temperature around 270℃ and a wet boiling point temperature around 170℃.The boiling point temperatures of DOT4 is very similar to those of DOT5.1. The major difference is the viscosity at low temperatures.
Today, cars are commonly equipped with ABS, and DOT5.1 fluid is used since the viscosity of it helps the ABS work consistently even in cold climates.

Most racing brake fluids are developed for circuit-use only. The boiling point temperatures easily exceeds DOT5.1 specification but the viscosity and pH levels do not pass DOT5.1 specification. This is why racing brake fluid do not pass DOT 5.1 specification.

The answer is simple. The technical difficulties to develop such a product will significantly increase cost. To develop a brake fluid with DOT4 spec viscosity, pH level higher than 7.0, and dry boiling point temperature above 320℃ can be very difficult, which would significantly increase production cost.

The use of racing brake fluid that exceeds DOT specification is safe for street-use. The use of racing brake fluid that do not meet DOT specification can speed up the deterioration of brake components over a extended period time. It can also lead to the malfunctioning of ABS during cold weather.

There are no problems with using racing brake fluid, if you are going to change fluids from circuit-use to street-use. For drivers who do not want to change brake fluids between the two uses, we recommend the use of racing brake fluid that meet DOT specification, DOT5.1 or super DOT4 brake fluid.

For everyday drivers, who use brake fluid that exceed DOT4 specification, fluid replacement once 2 years is acceptable. For people who use DOT3 fluid, fluid replacement every year is recommended. For people who drive aggressively on winding roads, fluid replacement every six month to a year is recommended. For people who drive their cars on the race circuit, replacement before each event is recommended.

Meet DOT5.1 boiling temp. specification but, low temp. viscosity characteristics is DOT4.

No, low temp. viscosity may soft but, will not soften pedal touch at circuit. The fluid temp. during circuit run become over 150℃ and viscosity characteristics at the temp.
range has no major difference from other grades. Also, there is no major interrelationship between fluid's adhesive (viscosity) and pedal touch.
The pedal touch depend on the moisture absorbed and lower the moisture volume more solid touch and higher the moisture volume softer (sponge like) touch.

Basically, not recommended.
If either fluid as glycol base and both passed DOT approved, no major functional problem. However, in performance-wise not become a average of 2 but, lower side kept.
Therefore, refrain from mixture of fluid but, replacement of whole volume is recommended.

For passenger cars, it generally requires 800ml ~ 1L for full volume replacement. (Trucks / Lorries are not in this category)
For partial replacement on Caliper portion alone, it is possible by 300~400ml