Agree with Gator, where are you mentors? They should be helping/guiding you through the design process of designing domestic water system.
Second, you would get more and better responses if this was posted in the Plumbing section.
Now about your questions.
1) It doesn't matter what type of sink (kitchen, lavatory, etc.). It is up to you to determine which row you use based on the sink you are specifying/selecting.
2) 20mm/15mm is referring to the faucet connection size.
3) Pipe sizes always reference the interior diameter. Note that this dimension is nominal, actual internal diameter is never this dimension exactly and varies depending on pipe material and gauge (wall thickness). There are various resources to get actual inside dimensions if you really need.
4) Sep./mix tap means that the fixture has 2 points of connection (one for hot water, one for cold) and the water is mixed internally in the faucet. Go to your local hardware store and take a look at the faucets on display.
Since the provided table does not include fixtures like water closets or wall hydrants, I assume it is referring to hot water demand values. In which case, capacity, flow rate etc. would be for the hot water side of indicated fixtures only. Read through the book (I know it's a foreign concept) you should find explanations for the table's content and how to use.
Being in the USA, I am used to working with "fixture units", a figure used in conjunction with Hunter's curve to estimate demand flow rate in gpm. The "Demand Unit" I assume is the SI equivalent to fixture units and be used with a SI calibrated Hunter's Curve (unless it goes by another name), consult with your mentors. Here in the USA we mostly use ICC IPC or some derivative which provides a tabulated form of Hunter's Curve.