The four criteria for assessing class importance apply to frontier works as follows:
- Period (currency): Extended. Frontier works dating to the medieval period were constructed for around 400 years from the 5th to the 8th century, although some may have been in use after this time.
- Rarity: Very rare. Only around 25 examples have been recognised at present.
- Diversity (form): Medium. Four main types of frontier work have been identified at present.
- Period (representivity): Very high. Frontier works represent one of a restricted range of monuments dating to the early medieval period.
Assigning scores to these criteria following the system set out in the Monument Evaluation Manual, Frontier works yield a Class Importance Value of 45. This lies two-thirds of the way up the range of possible values (max.= 64), reflecting the rarity and representivity of the class. Examples representing the full range of types of frontier work and of varying length should be included in the sample of nationally important sites.