Re templates, see:
https://support.office.com/en-us/art...2-1191BCE57DEB
https://support.office.com/en-us/art...A-A6F5E3E8B9AE
A mailmerge main document, on the other hand, is a document that has been connected to a data source for mailmerge purposes. Once you've completed its setup and saved it, it's suitable for running as many mailmerges as you need. You shouldn't have to recreate any field coding or filters each time, but you will need to answer to the SQL prompt you receive when opening the document.