This is the latest part of an ongoing project aimed at extending algebraic
properties of the classical modular group SL_2(Z) to equivalent groups in the
theory of Drinfeld modules. We are especially interested in those properties
which are important in the classical theory of modular forms. Our results are
intended to be applicable to the theory of Drinfeld modular curves and forms.

Here we are concerned with the cusp amplitudes and level of a subgroup of
such a group (in particular a congruence subgroup). In the process we have
discovered that most of the theory of congruence subgroups, including the
properties of their cusp amplitudes and level, can be extended to SL_2(D),
where D is any Dedekind ring. This means that this theory can be extended to a
non-arithmetic setting.

We begin with an ideal theoretic definition of the cusp amplitudes of a
subgroup H of SL_2(D) and extend the remarkable results of Larcher for the
congruence subgroups of SL_2(Z).

We then extend the definition of the cusp amplitude and level of a subgroup H
of SL_2(D) by introducing the notions of quasi-amplitude and quasi-level.
Quasi-amplitudes and quasi-level encode more information about H since they are
not required to be ideals. In general although the level and quasi-level can be
very different, we show that for many congruence subgroups they are equal.

As a bonus our results provide several new necessary conditions for a
subgroup of SL_2(D) to be a congruence subgroup. These include an inequality
between the index and level of a congruence subgroup.