It feels right to put format validation inside the domain objects (VOs or entities) because it is the natural place for high cohesion and the domain knows best what every domain description/attribute/property means. Many DDD practitioners and books authors (Vaugh Vernon, Dan Bergh - and even Eric Evans suggest , but on a different aspect: authorization, to model the domain to reflect these matters) sugestsuggest to design the domain model in a way to reflect and enforce a proper state of the business.
What about CQRS where the command side (where the domain model is) has little or no knowledge about the read side. Should one reimplement the same input format checks in the query side of the application? In CQRS there are some common responsibilities for both command and query side (like input format validation), so the usual format validation implementations that most of DDD practitioners sugestsuggest will be duplicated.
On the query side there might be an UI page where you can find all the scheduled conferences that have a certain number of available seats. So again , on the server's query side of the application, there must be a format validation that will check if the query for scheduled conferences with a number of seats available is a number and is in the range 1-100. This query might be needed by someone who wants to reserve a certain number of seats for a conference . Also conferences might be on the same topic so one could see a confenrececonference with the same topic many times and choose the one cheaper or with more seats available.
P.P.S. : Another question that popedpopped up : Does the query side of the application reuquirerequire any input format validation ? If the input is not a valid format it will return nothing , query found no data related to the request. I see that format validation on the query side is purely a security measure (ie. for buffer overflows). So is the input format validation really required on the query side ?