What is a mail merge credit in CRM?

What is a mail merge credit in CRM?

I have a question about word interpretation. The help in CRM says this:

Professional, Enterprise, and Ultimate edition users can perform 1000 merges/month/org by default.

What's a "merge" in this sentence? A merge action or a merged record? If I select 100 records in Zoho CRM and click the mail merge button, does that use 1 credit or 100?

On a side note, I would like to tell Zoho that the mail merge function is great, but it has limits that could be improved. I don't think I'm the only user that takes advantage of mail merge not to create customized letters, but to create reports.

Thanks!