Sub-verifiers can have child <Domain> elements that force them to be used only when the domain being queried matches one of the <Domain> elements. For example, a Multi verifier mapped to domains a.com, b.com, and c.com.
If it has two sub-verifiers, you could assign domains a.com and b.com to be handled by the first sub-verifier and domains b.com and c.com to be handled by the second sub-verifier. Then when a request to verify an email address in a.com is received, only the first sub-verifier will be consulted.
When a request to verify an address in the b.com domain is received, then both sub-verifiers will be consulted (in order). Lastly, emails in the c.com domain will only search the second sub-verifier. See Multi Verifier Example 2 for more information.