The {feedback} field is for use in your e-mail and SMS templates. When you use the {feedback} field in a template, ServiceM8 will generate a unique feedback link when you create an e-mail or SMS. When customers follow this link and provide feedback, it will be stored in the diary of the related item (e.g. in the Job diary if the feedback was related to a specific Job).
The general feedback link is a constant link for your ServiceM8 account. It can be used for customers to provide anonymous feedback not related to a specific Job. This link can be put on your website, invoice template or even your Facebook page. You can find your per-account feedback link in the Settings page under Feedback.