The source is mainly there for you to find the Analaysis marks for KAA which is different for each question marker (tends to be 2 or 3 quotes from the sources). They need to be relevant to the question being asked and you should integrate quotes of the sources to support your answer. Also, the source is usually there so that you can answer in context of the firm/industry/country. The source even aids you find points to write for your answer if you ever get stuck, but the main reason its there is your Analysis marks.
For example, a question asking the extent of cigarettes PED elasticity, you might quote '60% of cigaette smokers refuse to switch to vaping' from the text to show its addictive and hence cigarretes are more likely inelastic in PED. (You can evaluate later arguing against this)
Chains of analysis are what get you marks for Analysis on the KAA for longer questions. It basically means explaining step by step and in detail rather than simply stating your reason (which you might have to do for a 5 marker which has 1 mark for anlysis). The number of marks for analysis on each question is roughly how many sentences you should write explaining your point.
Hope that helps a bit