Hi there,
There are filters you can use to modify the email subject and message:
affiliates_new_affiliate_registration_subject
affiliates_new_affiliate_registration_message
The filters are documented on https://docs.itthinx.com/document/affiliates/api/filters/
This is for registration notifications. If you would like to customize the notifications related to referrals, you would need one of the premium versions, Affiliates Pro or Affiliates Enterprise.
Cheers