Major Topics are kind of like genres. One of the ways that they are useful is that they help subdivide articles into more categories. They can be particularly useful when you are trying to research an unusual aspect of a topic (such as if you wanted to analyse the pieces of a jazz composer, or if you wanted ethnographic information on a classical composer, etc.).
Using Major Topics in RILM is pretty straight forward and there are two ways to use them!
General Major Topics
One of the many features that the Advanced Search screen has is the ability to choose a range of generic Major Topics in an area.
The Major Topics that are available are listed under "Limit your results" in the box titled "Search Options" in this type of search
Specific Major Topics
Like many of the other skills on this tutorial, the easiest way to learn which specific "Major Topic" would work best for your research is to look at a record!
So the first step is to find the record of an article that fits your purpose. Now what you need to do is look at the field that says "Major Topics." You might notice it almost has things that seem like subcategories. Even if you end up not using Major Topics to search, they can help tell you if the item that you're looking at should suit your needs.
Now that you know which Major Topics look good you can use them in your search!
These can be searched by copying the desired string of text, or even just by using the number to the left of the topic (such as "81" which is "Music and related disciplines--Philosophy, aesthetics, criticism") and then pasting that into one of the empty search boxes that are in the Advanced search screen. Remember to change the "Select a Field" drop-down menu to "MT Major Topics."
Generally, it is best when you are trying out things like this to try to limit yourself to maybe only one of these (because you don't want to get so specific that you don't find any articles)!