The type of validity that McClelland is trying to establish is a form called predictive validity. Predictive validity is way of testing whether a score of a newly-constructed measurement predicts scores from another measure (a criterion) that has previously been found to be correlated with the constructed measurement. If the relationship is found in these two measurements, it can be said that the newly constructed measurement is valid.
Predictive validity is a part of the criterion-related group of validity, together with concurrent validity. The former differs from the latter in terms of when the criterion-measurement is administered – where if it is administered together it is concurrent validity, while if it is after it is predictive validity.