A reduced score is the
quick and dirty version of a full score. It is easier to produce, smaller, and more readily understandable by your average minimally educated
musician.
A reduced score is in
concert pitch, meaning that it is not transposed. A reduced score also features as few staves as possible, sometimes even having four independant lines or voices on one or two staves. You may find that a reduced score works best for your daily
theory exercises, but in serious
composition and score preparation, one will always want to use a
full score.