Accessio credit principali means that an addition to a principle becomes part of it. This is very much when you an extension to your house it becomes part of the house. However in all architecture IT related we sometime forget to add editions to the architecture. In most countries in the world building extensions is often the only way to build structures the would not be allowed in the first instance, but since extensions work on a less bureaucratic way the result are principal structures that should not exist.
The same is often also true for IT projects as extensions often completely change an architecture and as such create constructs that should never have been possible in the first instance. So this is why we need an architectural check on extensions to evaluate if an extension is in line with the original architecture or if the extension is actually something against the enterprise architecture or something that requires a new architecture. However for something like this check to work an architect will need to sit on the change board that is often avoided as pure tactical by many architects.