May play a role in vesicle-mediated protein trafficking to lysosomal compartments and in membrane docking/fusion reactions of late endosomes/lysosomes. Mediates phagolysosomal fusion in macrophages. Proposed to be involved in endosomal maturation implicating VIPAS39. In epithelial cells, the VPS33B:VIPAS39 complex may play a role in the apical recycling pathway and in the maintenance of the apical-basolateral polarity. Seems to be involved in the sorting of specific cargos from the trans-Golgi network to alpha-granule-destined multivesicular bodies (MVBs) promoting MVBs maturation in megakaryocytes (By similarity). Interacts with RAB11A and VIPAS39. Associates with adaptor protein complex 3 (AP-3), clathrin:AP-3 and clathrin:HGS complexes (By similarity). Colocalizes in clusters with VIPAS39 at cytoplasmic organelles. Colocalizes with RAB11A and VIPAS39 on recycling endosomes. Phosphorylated on tyrosine residues. Belongs to the STXBP/unc-18/SEC1 family.