a shorthand for custom validators
Instead of using the fully qualified classname (which is long) Extbase has a shorthand of the form <ExtName>:<ValName>. This will resolve to Tx_<ExtName>_Validation_Validator_<ValName>Validator there.
For FLOW3 instead of <ExtName> the <PackageName> could be used to create a class name.
(Thanks go to Martin Kutschker for that suggestion.)