The first thing is Regularity, which means for the RBox Axioms there are certain Restrictions concerning the Interactions of RBox Axioms that deal with Regularity. There are only certain forms/schemata of Role Inclusions allowed and there must be a strict order to be maintained. On the second hand one has the simplicity of Properties that must also be maintained because there are several Restrictions on which Properties, e.g. in Number Restrictions are only applied on which other simple Procedures and other Things are applied, only on simple Properties and not on Properties that relay on General Property Inclusion, otherwise one would lose the Property of decidability.
One has to be carefully as it might be that the Union of several Knowledge Bases violate these Restrictions, although each single Knowledge Base does comply to the Restrictions.
To ensure decidability the following structural Restrictions must hold for SHROIQ(D):
Restriction of potential interaction of RBox axioms