The brass section of a drum corps is the group of individuals who play brass instruments. In the past, only bugles were allowed according to competitive rules, but now drum corps are free to use any acoustic bell-front brass instruments, with no regard to their fundamental pitch (key) or classification as bugle.
The subsections of the drum corps brass section are generally identified by their note range: soprano, alto, tenor and bass.
The first bugles used by civilian drum & bugle corps were US military field trumpets, as they were dubbed, because they used a bore more similar to a trumpet than the older bugles used previously. They had no valves or crooks of any kind to adjust pitch. These were also the first bugles to be standardized to the key of G, and then only within the United States.
These bugles were used, alongside drums, in the first drum & bugle corps established during the Civil War as signaling units. Upon the advent of the radio as an instrument of communication, the military drum & bugle corps became obsolete for its primary purpose. The government sold or gave their surplus drums and bugles to veteran's outposts for them to use for their own drum & bugle corps, for civic events and other uses. As time went on, more drums and bugles were produced and churches, scout troops, YMCAs, and a wide variety of other organizations started their own corps.
As more corps were added, competitions began to be established, and ruling bodies set up rules to govern competitions on a national level.
Over time, individuals and groups began to experiment with bugle designs and produced bugles capable of playing more than just partials. A slide was added first, then a valve. The valve was horizontal, and most rule-making bodies insisted upon its addition to the bugle that it be locked either open or closed. This made for horn arrangement that somewhat resembled modern bass drum arrangement, with notes alternating between the "up" and "down" bugles. Later this requirement was removed, and the valve could be operated by the thumb.
In the 1950s a second valve was legalized. However, the rule established allowed only one valve to be vertical. The design that came about left the horizontal piston valve in place to be operated with the right hand, and added a rotor valve that was operated by the left hand. Often rather than purchasing new instruments, the second valve could be purchased from the manufacturer and added on by using it to replace a slide on a single-valve bugle.
Having two valves opened up many possibilities for arranging for hornlines. Rather than trusting notes outside the normal partials to a player's ability to bend the pitch, arrangers could write much more creatively and not worry about a hornline's ability to play a part.
Over time more problems arose, however. Bugle manufacturers began to close their doors or stop producing bugles. To deal with this, DCI legalized bugles with two upright valves. This allowed instrument manufacturers to use some parts that were interchangeable with their normal band instruments in the manufacture of bugles.
In 1990 DCI legalized the third valve, finally allowing junior corps to use fully chromatic instruments. The first corps to field a hornline with all three-valved bugles was the Bluecoats. A fourth valve was also legal for contrabasses, but was not in wide use or production until the late 1990s.
In 1999 DCI voted to allow bugles in any key to be fielded, and DCA followed suit in 2003.
The soprano bugle provides much of the melody in a drum corps show. In some cases, flügelhorns are used for a warmer sound within the soprano range.
The alto section consists of mellophones and occasionally French horn bugles. Mellophones are the most common instrument, due to its greater suitability to modern drill requirements than the marching French horn. Some hornlines also use marching versions of alto horns.
The baritone and euphonium provide music in the tenor range. In reality, both are actually euphoniums as euphoniums are defined, as they have conical bores. (True baritones, as seen in a British brass band, have cylindrical bores) Baritones are usually the smaller of the two, with smaller bores than marching euphoniums. Different hornlines use baritones and euphoniums in varying combinations, depending on what color the instructors want out of the hornline.
Both of the above horns were later additions to drum corps hornlines, as corps began to use bugles manufactured specifically for them. The first alto and tenor bugles were simply larger versions of the soprano, and over time have evolved into the instruments they are today.
The contrabass provide the lowest sounds on the field. The first contra was developed by Whaley Royce in the 1960s, and is the only horn in the hornline to never have a predecessor with a single valve or no valves. It is also the only instrument to be carried on the shoulder due to size, and the only instrument to have four valves in its modern incarnation.
Information on brass arranging can be found by following that link.