Context aside, we should consider the template for answering this question first. At least two marks will be awarded immediately for simply stating the origin and purpose of the sources.
For example source A is 'a letter written by the German chancellor Prince Max of Baden in October 1918.' Although this should be written out in the source, paraphrasing is a good idea to show the examiner that you are not just blindly copying the source. The purpose of the source was for the German chancellor to negotiate a peace for the First World War on American terms, rather than on British or French terms. This goes beyond simply stating that 'The Germans tried to negotiate peace with America'. It makes an analytical point about its purpose. This sort of analysis is required for the top band marks.
The second stage to this question is to assess the value and limitations. Attempt to make between 3 and 4 points. Do not go under or you will lose marks. There's is not much point in going above 4 as well, as you would just be wasting your exam time. You can have more value points or more limitation points, as the IB does not expect a 50:50 split. But do try to make at least one point about the source's value, and one point about the source's limitation. These points can be found beyond the source in its origin or purpose. For example a value of the source is that it was written by the German chancellor Prince Max of Baden, which tells us that it was the German politicans who were attempting to negotiate peace rather than the German military leaders. A limitation of the source is that it doesn't tell us whether the Fourteen Points were the agreed method or not used to bring about the Armistice in 1918. Or it can be found within the source. Often the key point here is in the nature of the language. For example if it was written in a rhetorical tone, then it may have been used for propaganda purposes, and we can't be sure of the validity of all the facts.
Finally, as a last recommendation, use the words 'origin', 'purpose', 'value' and 'limitation' within your essay, as it provides an easy signpost for an examiner so that they don't miss any of your points.