The merge fields that can be used in a template depend on the folder that the template is saved in.
For example, a template saved in the Batch folder can access merge fields from the Finance module. A template saved in the Events folder can access merge fields from the Events module.
Field Rules allow you to use merge fields that would not normally be available. This could be certain Merge Fields from another module or special merge fields that contain calculations. For example, last donation.
In the example below, I wish to include the donor’s last donation amount in my next appeal letter. The Appeal Letter is stored in the Contact folder, so a field rule will be used to access the last donation amount field from the Finance module.