When a blogger enable E-mail Notification for comments on his/her blog, the following options are available:

HTML - the notification will be sent in HTML format, the message will contain URLs to the post and comment API. Reply to this mail will add new comment, if mail-gateway is enabled by Web administrator.

HTML Form - this option will send an email containing comment and form, which could be used to invoke directly comment API from mail agent (if it supports posting a form)

Text - this option is same as first, but uses plain text instead of HTML encoding.

combined : combines all of the above as attachments.