It depends on the source of the data for the merge fields.
When working in the templates within the CRM, Transactions & Invoices, Event, and Membership modules, they have their own list of mail merge fields available and are drawn from the main grids within those modules. For example, in the CRM templates, fields from the Constituent Grid are available to be inserted into the template. The available merge fields are shown alphabetically in the list when you select Insert Merge Fields while working in a template.
When working in email marketing campaigns, how the contact email list was created will determine what data is available for the templates. This also applies to acknowledgement emails for things like membership, event, or auction page transactions. For example, an email list created by filtering an event attendance grid will provide different merge fields than one created from the CRM grid. Or, as in the example below, an auto reminder set up for membership renewals will have merge field data related to memberships.