Non-functional simulation (testing) targets non-functional requirements: the way a system performs, rather than specific behaviors. This is in contrast to functional testing, which targets functional requirements that describe the functions of a system and components. The names of non-functional tests are often used interchangeably because of the overlap between various non-functional requirements.
For example, performance is a broad term and will have coverage for attributes like reliability and scalability.
Non-functional testing is concerned with NFRs and is designed specifically to evaluate the readiness of a system according to the various quality attributes which are not covered by frequent functional testing. For example, in functional testing, it might be revealed that the function of inputting data into a set of cells within a database works, but usability testing (a part of non-functional testing) shows that persisting a version of the document requires two minutes...