  The vector magnetic field characteristics of superactive regions (SARs) hold
the key for understanding why SARs are extremely active and provide the
guidance in space weather prediction. We aim to quantify the characteristics of
SARs using the vector magnetograms taken by the Solar Magnetic Field Telescope
at Huairou Solar Observatory Station. The vector magnetic field characteristics
of 14 SARs in solar cycles 22 and 23 were analyzed using the following four
parameters: 1) the magnetic flux imbalance between opposite polarities, 2) the
total photospheric free magnetic energy, 3) the length of the magnetic neutral
line with its steep horizontal magnetic gradient, and 4) the area with strong
magnetic shear. Furthermore, we selected another eight large and inactive
active regions (ARs), which are called fallow ARs (FARs), to compare them with
the SARs. We found that most of the SARs have a net magnetic flux higher than
7.0\times10^21 Mx, a total photospheric free magnetic energy higher than
1.0\times10^24 erg/cm, a magnetic neutral line with a steep horizontal magnetic
gradient (\geq 300 G/Mm) longer than 30 Mm, and an area with strong magnetic
shear (shear angle \geq 80\degree) greater than 100 Mm^2. In contrast, the
values of these parameters for the FARs are mostly very low. The Pearson
\c{hi}2 test was used to examine the significance of the difference between the
SARs and FARs, and the results indicate that these two types of ARs can be
fairly distinguished by each of these parameters. The significance levels are
99.55%, 99.98%, 99.98%, and 99.96%, respectively. However, no single parameter
can distinguish them perfectly. Therefore we propose a composite index based on
these parameters, and find that the distinction between the two types of ARs is
also significant with a significance level of 99.96%. These results are useful
for a better physical understanding of the SAR and FAR
