What enables us to send the same letter to different person in Microsoft word?

Mail Merge.

In the multiple documents created at once, they will have the same graphics, text, and formatting. However, certain sections of the document can be different by them being personalized. Along with letters, mail merge can also include labels, emails, and envelopes.

Three documents are included in the process of mail merge, which are the the primary document, data source , and the document that is being merged. Preparing the data is the most important part of mail merging, because the data source is used to identify the list of recipients. Mail merge is a feature of Microsoft programs including Word and Excel.