In almost all cases NewInstancePerTarget (the default) would fit. In such case every target gets a new instance of the instance type injected.
In Singleton, every target gets the same instance of the instance type injected. This will be useful if, for instance, you have an Auditor or Validator that writes directly in a single text file.
If you are using Validators with straight forward validation I think NewInstancePerTarget (the default) is recommended.
