In mail merge, you can upload new files and these files should be either excel files or csv files.
These files should have at least contact numbers to which messages have to be sent.
New files can be uploaded as shown in the above picture.
Once you have uploaded your files you can view all these files. Now these files can be used in your mail merge.
one of the files has been used from the list of mail merge files to send messages using mail merge.
You can insert columns of your excel files in the message which will be inserted like %Customername% and doing this way you will be able to send customise messages to your clients
What is Insert Column? : This works like Mail Merge. Assume you have Client Name, Invoice Number and Invoice Amount in the Excel file then you could write your message like this Hello %Client Name% your Invoice Number is %Invoice Number% and amount is %Invoice Amount% hope to receive your payment on time. Thanks Your Company Name.