Short answer: There is none.

SEM stands for Search Engine Marketing or sometimes Search Engine Management. It is most frequently used to reference the paid advertising opportunities on search engines.

Rarely, when SEM is used to mean Search Engine Management the usage my infer both SEO and PPC Advertising. This was far more common in the early days of search engines and this usage is seldom seen in modern digital marketing. If a freelancer or agency is selling you SEM used as Search Engine Management, you should clarify what type of search engine marketing they are referring to.