what is functionality testing?

Functionality Testing:
Functionality testing or requirements coverage
1. GUI or behaviral testing
Graphical user interface and navigatinal testings
2. Input domain testing
Input data testing based on BVA and ECP
BVA: Boundary value analysis
Range and size
ECP:Equivalent class partition
valid data and in valid data.
3. Error handling testing
How the Error messages are managed at appropriate places and valid matter displayed in them
4. Back end testing
what we are entered in the front end is stored in back end and retriving the same.
5. Mutation testing
If any changes made is it showing the changes.
here we can say it as negetive testing.
6. Sanitation testing
un wanted fields or functionalities which create ambigvity for the user are checked