It can tell you whether their DNA is consistent with a single breed. If the report shows almost 100% of one breed, this is consistent with purebred ancestry in recent generations.
However, it is not a certificate:
• Purebred status is documented by a pedigree, issued by a recognised breeding organisation.
• Some breeds are very similar genetically and can be difficult to distinguish.
• A small percentage of another breed may reflect historical crossbreeding or a related breed, rather than a recent cross.