This article is from the Drum corps FAQ, by Cathy Doser firstname.lastname@example.org with numerous contributions by others.
For competitive purposes (i.e. at DCI competitions), a bugle is
defined as follows: "By the word `bugle' as used herein is meant a
straight bell-front brass instrument pitched in the key of G. All
instruments shall have no more than three (3) valves except for the
contrabass bugle, which may have four (4)."
This is quoted from Drum Corps World (January, 1990) which quotes
the rule book (Rule 4.3.1). This rule was passed at the 1989 DCI
Rules Congress. The rule used to say exactly the same thing, except
that all instruments (even contrabasses) could have only two (2) valves.
There is an interesting history to what was allowed on the field.
At first only valveless bugles were allowed. Then one-valved, piston-rotor,
and then two-valved were allowed, in that order. In fact, there used to be
only soprano bugles until the baritone bugle, mellophone bugle, French horn
bugle, and the contrabass were invented.