“Traceability (or Requirements Traceability) refers to the ability to link product requirements back to stakeholders’ rationales and forward to corresponding design artifacts, code, and test cases.” (quoted from Wikipedia)

Full-scale traceability

Full-scale traceability needs every single requirement to be referenced in any part of the architecture/solution documentation.

Pragmatical traceability

From a more pragmatical perspective, traceability can also refer to the understandability or explanation of important or fundamental architecture or solution decisions: “We took the decision Dec-X because of the requirements Req-Y.”