What is sphere of influence?

The sphere of influence refers to the condition in which one country has power to control another (economically, developmentally, etc.) without being officially recognized as the sovereign authority.
The concept was important in the 1880s when the Western European countries were establishing colonies in Africa and Asia. Although these colonial powers were all competing for territory, they agreed to do so peacefully. England and Germany first used the term in 1885 in their agreement to recognize “a separation and definition of their respective spheres of influence in the territories on the Gulf of Guinea.”
This term also refers to relationships on a much smaller social scale, especially in marketing jargon. People tend to value their friends’ and family’s opinions more than impersonal advertisements. Your own “sphere of influence” includes everyone you know who would consider buying something if you gave it a favorable review. Marketers know that as traditional advertising techniques grow less effective, these individual “brand advocates” are vital for the success of a product. This is why businesses are always trying to get everyone to interact with and share their social media posts: it’s easy to ignore an ad, but if your friends think you like something, they’re more likely to pay attention.
https://www.britannica.com/topic/sphere-of-influence