The purpose of this study is to compare the effect of ixazomib+dexamethasone (ixa+dex) versus pomalidomide+dexamethasone (pom+dex) on progression-free survival (PFS) in participants with relapsed and/or refractory multiple myeloma (RRMM) who have received at least 2 prior lines of therapy, including lenalidomide and a proteasome inhibitor, and are refractory to lenalidomide but not refractory to proteasome inhibitors.