difference between preapproval and terms approval serial

Summary: What is the major difference between Preapproval serial and Terms approval for the Purchasing Document

Content (please ensure you mask any confidential information): It was suggested for me to setup the approval configuration under Preapproval and terms approval simultaneously before that need the difference between both.

Version (include the version you are using, if applicable):

24B

Code Snippet (add any code snippets that support your topic, if applicable):