Files with dia file suffix are related to the Psion Organiser and used for its diary data.
DIA file extension - Psion Organiser diary data
What is dia file? How to open dia files?
File type specification:
The dia file extension is associated with the Psion Organiser CM/XP Format for OPL (Organiser Programming Language).
OPL is a quite powerful compiler language. This means that the source code (what you type in) is first translated into a kind of machine code and can then be used like a built-in application.
The dia file stores data for Psion Organiser diary.
This dia file type entry was marked as obsolete and no longer supported file format.
This type of file is no longer actively used and is most likely obsolete. This is typically the case for system files in old operating systems, file types from long discontinued software, or previous versions of certain file types (like documents, projects etc.) that were replaced in higher versions of their original programs.