Functionality is concerned with looking at if the outcome actually does what it should. The functional requirements will often be listed in the design specification outlining exactly what the outcome needs to be able to do.
Questions that you should consider are:
- Does it work well?
- Is it efficient?
- Are there any problems / bugs?
- Will it work on a range of devices?
- Do all aspects of the outcome work (links, buttons, images, etc.)?