Mappings are one of the most common scenarios. Can you get any more specific in your question?
Have you looked at Anapedia: https://help.anaplan.com/anapedia/Content/Calculation_Functions/All/LOOKUP.html
or the Micro-Lesson: https://community.anaplan.com/t5/On-Demand-Courses/The-LOOKUP-Function/ta-p/64232
or When to use Sum vs Lookup: https://help.anaplan.com/anapedia/Content/Calculation_Functions/Function%20Usage/Formula%20Usage%20T...
Hopefully, these resources will answer your question.