You can edit Email Templates to include selected fields from the database as follows:
Via the main menu select, System, Email, Templates
Select the applicable email template you wish to edit
Click Edit
Click on Add Field
Select which data field you wish to add.
This adds a field code, e.g. [[Supplier.Code]], at the bottom of the template.
Select/highlight the field code including the brackets,then cut and paste the field code into the required location on the template. This can include the Subject line if required
Click Save
Once you have applied it - it is worth testing on just one document or transaction first to ensure it appears as required and adjust it by editing it again if necessary.