Functional bugs are related to a functionality. If a button doesn’t work, the search doesn’t react to the user input, if an app crashes or a product detail page isn’t or not properly displayed – it’s a functional issue. Every time you click and the website/app doesn’t respond as it should, it’s probably a functional issue.
If there are elements out of place or pictures with bad quality, it is not a functional bug.
Functional bugs have three priorities:
Low: Minimal impact on user experience
High: Serious impact on user experience, but doesn’t prevent the function of the app or website.
Critical: Preventing a function of the app or website, Causes a potential loss of income for the company running the app or website. e.g. an app crash.
Critical issues like crashes etc. should contain crash logs as part of the documentation.