DOT ratings and Boiling points
The DOT number rating relates to the standards set by the American Department Of Transportation.(DOT). Mainly, these relate to the fluids boiling point, in so much as this represents the single most important characteristic of the fluid and it states the boiling point required to meet the different standards.
The problem is that that boiling point degrades with time due to the fact that brake fluid is Hydroscopic. This means that it attracts water. In fact, its affinity with water is so pronounced that even fluid in the system absorbs it, albeit very slowly right through the micro pores present in the hose and other components.
Therefore, in time the fluid in every braking system inevitably absorbs water or becomes (wet) which in turn causes a significant decrease in the fluids boiling point and therefore its performance.