What is the behaviour of validations during a dimension import?

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Hi,

When performing a dimension import, no validation errors are thrown. But on review and validation from the viewpoint I get a large number of validation errors. Why was this not picked up during the import and terminated? I don't want to import invalid data. 

What am I doing wrong or is this behaviour by design.

Is this the same for System, Application and custom validations?

Thanks

Simon