Tableau, data visualisation and story telling group and set

what is the difference between group and set in tableau?and how to know where we should choose group or set?

Grouping in Tableau is grouping multiple members in a SINGLE dimension into a higher category and creating a set is grouping members from MULTI dimensions and/or condition into a dynamic or constant group . So sets are just a more complex and dynamic grouping.
for more details i suggest you to go through this blog

thanks for your reply.