Difference between design specification outdoor air and space infiltration design flow rates

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Hi there,

I am trying to model a residential building in OpenStudio and am on to infiltration but I am very confused what the difference is between design specification outdoor air and space infiltration design flow rates. Would someone be able to explain this to me?

For the residential building I know ACH should be 0.25 but am unsure which infiltration category this comes under, if someone is able to clarify?

Thanks.