One can specify a volume as to indicate gallons of gasoline or diesel as opposed to gallons in the abstract, or to indicate joules of mechanical energy rather than general energy. The effect of this is that the annotated units do not recombine with un-annotated units. This is nice for discussing fuel consumption, for example: without this feature, the unit mile_per_gallon would simplify to approximately , since its dimension is ; however, does not simplify (its dimension is ).
One obtains an angle in radians by dividing the length of an arc by the length of a radius. At first sight, this would seem to require that an angle always simplifies to unit 1, which would be undesirable. The solution is that in Maple, the denominator gets a unit of the dimension ; for example, . So a radian is defined as a .
As an example of contexts, Maple understands a mile[standard] as different from a mile[US_survey]: one standard mile is US survey miles. Or for a different example: minute can refer to a unit of time, but also to of a degree - the unit of angle. In Maple, these are known as the minute[SI] (or just minute, since SI is the default context for minute, as can be seen on the help page Units,time) and the minute[angle], respectively.
If both a context and an annotation are needed on a unit, then the context should be specified first. For example, a circle segment with a radius of and an arc length of describes an angle of .