Its the field where USe case are studied, requirements are studied and documentation issues are found, before even the testing begins.
There are some threads in forums on the same topic. Plz do a search.
There is no such thing called as "Design Assurance Testing" in the Software Field. Well, to check the "rugged" behavior of electronic goods, one may do design assurance on the circuit boards, but there is no specific engineering discipline called as "Design Assurance Testing". When a software product or application is designed, the design can be validated for the "future proofing" capabilities to assure its strength.
Just googled Design Assurance testing and I did find it being used for electronics. I am sure it could also be a term used in software testing as it is not just the software or hardware we test but also the design and function. BY the way Rich W, love the Mona Icon!!!