A Study Designed to Evaluate the Safety and Efficacy of Venetoclax Plus Dexamethasone (VenDex) Compared With Pomalidomide Plus Dexamethasone (PomDex) in Participants With t(11;14)-Positive Relapsed or Refractory Multiple Myeloma.

* Documented diagnosis of multiple myeloma (MM) based on standard International Myeloma Working Group (IMWG) criteria. * Measurable disease at screening as defined per protocol. * Has received at least 2 prior lines of therapy as described in the protocol. * Has had documented disease progression on or within 60 days after completion of the last therapy. * Has received at least 2 consecutive cycles of lenalidomide and be relapsed/refractory to lenalidomide, as defined per protocol. * Has received at least 2 consecutive cycles of a proteasome inhibitor (PI). * Has t(11;14)-positive status determined by an analytically validated fluorescent in situ hybridization (FISH) assay per centralized laboratory testing. * An Eastern Cooperative Oncology Group (ECOG) performance status less than or equal to 2. * Laboratory values (liver, kidney and hematology laboratory values) that meet criteria as described per protocol.

* History of treatment with venetoclax or another B-Cell Lymphoma (BCL)-2 inhibitor or pomalidomide. * History of other active malignancies, including myelodysplastic syndromes (MDS), within the past 3 years (exceptions described in the protocol). * Evidence of ongoing graft-versus-host disease (GvHD) if prior stem cell transplant (SCT). * Prior treatment with any of the following: allogeneic or syngeneic SCT within 16 weeks prior to randomization; or autologous SCT within 12 weeks prior to randomization. * Known central nervous system involvement of MM. * Concurrent conditions as listed in the protocol.