The A in DITA

DITA provides a standardised architectural framework for creating and maintaining content.

DITA can be described as an approach to information architecture. That's why there's an A for Architecture in DITA!

A great definition of architecture in the DITA context can be found in the book DITA 101:
An open content standard that defines a common structure for content that promotes the consistent creation, sharing, and re-use of content.
Ann Rockley, Steve Manning, and Charles Cooper
This definition is great because it highlights the facts that: