Why are some letters reserved?
The Arduino IDE does keyword coloration according to a very crude system. It simply scans for all the keywords.txt files included with:


Arduino IDE
Libraries bundled with the Arduino IDE
Libraries you have installed to the libraries subfolder of your sketchbook
Libraries bundled with the hardware package of the currently selected board - Hardware package of the currently selected board (recent IDE versions only)


There are several types of keyword identifiers that determine which color is used. The reason those letters are colored in your IDE is because some library you have installed just happens to have added them to its keywords.txt file.
The Arduino IDE makes no attempt to determine whether you have included those libraries and so coloration doesn't necessarily mean they are "reserved". If you have not included the library that defines a specific keyword then the coloration of that keyword is meaningless.

For more information on keywords.txt see:
https://github.com/arduino/Arduino/wiki/Arduino-IDE-1.5:-Library-specification#keywords