Strongly typed data descriptions are defined with a type declaration which contains the STRONG phrase or are subordinate to a type declaration with the STRONG phrase. Only group items can be strongly typed as the concept of strong type definition for elementary items would entail a large number of serious restrictions.
In addition to the protection of the data structure which is defined by the type, strong type definition is designed above all to protect the integrity of the data content. The correct use of strongly typed data descriptions should therefore never lead to data which is incompatible with its data description.
An important consequence is therefore to forbid any implicit or explicit redefinitions.
A group item can be regarded as a new definition of the elementary items which are subordinate to it. Consequently accesses to strongly typed group items are restricted to those which do not affect the integrity of the data which is subordinate to the group item in question. If a strongly typed group item is a receiving item, the sending item must be of the “same type”. The “same type” is defined as a type description with the same name and the same relevant characteristics.
There is no need to require the same restrictions for elementary items since accesses to elementary items do not generally corrupt the content of the receiving item.
Further restrictions exist for strongly typed group items and elementary items which are subordinate to strongly typed group items. To guarantee data integrity they cannot be renamed (RENAMES clause). Reference modification is only permitted for alphanumeric or national data items.
Restrictions for the addresses of strongly typed group items and pointers containing such addresses are described in the section "Type-specific pointers".
You should be aware that protecting the integrity of data is only meaningful if the data is above all “correct”.