Salesforce: Workflow - Evaluation Criteria

Workflow is one of the feature used most in Salesforce, because of the simplicity and it is so powerful. With workflow, administrator can defined Immediate or Time-Dependent Workflow Actions to create new Task, send Email alert, field update and send Outbound Message without the need of a developer to write code.

You need to define Evaluation Criteria correctly to fit your business requirements:

created
This option is pretty clear, workflow will be evaluated only when user create a record.

created, and every time it’s edited
For this option, workflow will be evaluated when user create new record AND anytime user edit the record.

created, and any time it’s edited to subsequently meet criteria
For this option, workflow will be evaluated when user create new record AND anytime it hasn't trigger. Example:
- Workflow with rule criteria Execution Time equals null AND Request Time not equal to null
- User create new record with both Execution Time and Request Time is null, workflow will not be triggered as not meet rule criteria
- User update Request Time not to null --> workflow trigger
- User update other field, it still meet rule criteria, but workflow will be not evaluate again, because it has been evaluate when user update Request Time not to null.

So, how/when workflow will be evaluate again?
- User update back Request Time to null OR Execution Time not to null, here rule criteria broken.
- User update Request Time not to null OR Execution Time to null, workflow will be evaluate again.