In order to assign automated 2.1.12 Area Classification to all of your accounts you have to configure the correlation between zip code and code in the object area classification. To do so go to the app launcher and search for area classification. Create new records for all your codes:

Enter 

• the country
• Zip range from - to (or use the same ZIP code in the from and to field) and make sure that the strings match exactly (Case-Sensitive)
• Area Classification String

A record with the following data 

will lead to the classification of 3a for this account: