What are supporting sentences?
The supporting sentences of a paragraph develop the main idea presented in the topic sentence. This development can come in the form of examples, reasons, description, etc., depending on the purpose of your paragraph. If, for example, you are writing an opinion-based paragraph, your supporting sentences will consist of reasons and details. If, on the other hand, you are writing a description, your supporting sentences will contain specific details to help the reader form a mental image.
Unity
It is essential that your supporting sentences stay on topic and clearly relate to the main idea of the topic sentence. This connection between the supporting sentences and the topic sentence is called unity. If your supporting sentences are irrelevant or off-topic, your paragraph will not be strong and may be unclear.
Review - Unity in English Academic Writing