In Excel the sum B2 and B6 simply means that B2 and B6 Numbers are adding to get a Sum, it actually like the normal Calculator, but in Excel it helps to do the calculation of your your data simple and easy, there is no hard work.
C. Let the database assign a unique number in a new field
I’m not to sure what this is asking, if by selecting word- it means the word is already left-click selected, so it would be option 3. If not, then it would be option 2 I believe