/IgnoreIsLastDcInDomainMismatch:{Yes | } default causes the wizard to prompt the user to continue and causes the command-line ...

/IgnoreIsLastDcInDomainMismatch:{Yes | } default causes the wizard to prompt the user to continue and causes the command-line tool to exit with an error. Specifies whether to continue the demotion of the domain controller when either the switch /IsLastDCInDomain:Yes is specified and dcpromo detects that there is actually another active domain controller in the domain, or when the switch /IsLastDCInDomain:No is specified and dcpromo cannot contact any other domain controller in the domain.