Solids elements should only be used when the thickness of an element is significant compared to it's plan dimensions. Typically for modeling beams/columns you would use beam elements and for modeling elevated slabs/foundation mats/footings you would use plate elements. Solid elements could be used for modeling very thick concrete masses as in concrete dams or for modeling very thick foundations like block foundations for vibrating machines or turbo generator foundations etc.
The following wiki would offer some guidelines
https://communities.bentley.com/products/ram-staad/w/structural_analysis_and_design__wiki/8445/member-plate-and-solid-which-one-to-use