Can any one post me the Hierarchy of salesforce Tags

I'm trying to create a worfklow rule using the criteria below

I want to send an email to the account owner as soon as the status of the "orders blocked"-picklist field is set to "Yes" and the previous value was equal to "No" or "Yes/No".

I thought this was an easy one but apparantly this kind of formula is not possible in Salesforce?

Can anybody explain me why not, and how to formulate this criteria rule so that it doesn't give an error message?

Thanks in advance.