Major supporting details have essential information that explains the main idea. Minor supporting details expand on this information and provide more detailed points that are not necessary for the reader to understand the main idea. There is no definite rule for how many of each detail type is contained within a paragraph.
Major supporting details provide the basic, necessary information for the reader to understand the topic of the paragraph or text. These can be viewed as the essential outline of the topic that the reader can focus on to fully understand the main idea if he does not want to delve deeper into the specific details. There are usually at least two or three of these per paragraph.
Minor supporting details are more specific and offer superfluous information that the reader does not need to know. This information can be overlooked if the reader is not interested in such specific details. Minor supporting details reinforce key points that come up within the major supporting details and dig deeper into the topic.
Both major and minor supporting details are necessary to fully build the topic, and the paragraph's development depends on these details, although major ones are more essential. Supporting details in general support the main topic sentence of a paragraph.