Plays a role in vesicle-mediated protein trafficking to lysosomal compartments including the endocytic membrane transport and autophagic pathways (PubMed:25273556, PubMed:26783301). Believed to act as a core component of the putative HOPS and CORVET endosomal tethering complexes which are proposed to be involved in the rab-5-to-rab-7 endosome conversion probably implicating sand-1, and via binding SNAREs and SNARE complexes to mediate tethering and docking events during SNARE-mediated membrane fusion (By similarity). The HOPS complex is proposed to be recruited to rab-7 on the late endosomal membrane and to regulate late endocytic, phagocytic and autophagic traffic towards lysosomes (By similarity). Within the HOPS complex, contributes to the normal development of gut granules in intestinal cells of the embryo, and also promotes the trafficking of embryonic intestinal gut granules away from lysosomes (PubMed:24501423, PubMed:25273556). The CORVET complex is proposed to function as a rab-5 effector to mediate early endosome fusion probably in specific endosome subpopulations (By similarity). Required for fusion of endosomes and autophagosomes with lysosomes (PubMed:18923146, PubMed:26783301). Plays a role in the degradation of apoptotic cells during programmed cell death (PubMed:18923146). Probable core component of at least two putative endosomal tethering complexes, the homotypic fusion and vacuole protein sorting (HOPS) complex and the class C core vacuole/endosome tethering (CORVET) complex. Their common core is composed of the class C Vps proteins vps-11, vps-16 and vps-18, which in HOPS further associates with vps-33.1, vps-39 and vps-41 and in CORVET with vps-8 and vps-33.2. In hermaphrodites, expressed in coelomocytes and gonadal sheath cells. Ubiquitously expressed in embryos. In early larvae, expressed in hypodermal cells, seam cells and body wall muscle cells. Temperature-sensitive defects in the formation of gut granules during embryogenesis (PubMed:24501423). At 15 degrees Celsius, pretzel-stage embryos have a reduced number of gut granules in intestinal cells, and at 22 degrees Celsius, pretzel-stage embryos completely lack gut granules in intestinal cells (PubMed:24501423). At 25 degrees Celsius, many embryos arrest by the pre-bean stage before elongation, and 76% of these embryos contain gut granules irregularly distributed throughout the embryo (PubMed:24501423). Defective apoptotic germ cell corpse digestion with delayed degradation of chromatin in late germ cell corpses (PubMed:18923146). This results in increased numbers of germ cell corpses at 20 degrees Celsius during embryogenesis and post the L4 stage of larval development, and furthermore the retention of cell corpses for a longer duration of time (PubMed:18923146). Impaired formation of endosomes and lysosomes in coelomocytes, in particular there is impaired formation of recycling endosomes (PubMed:18923146). In addition, there are endosome/lysosome fusion defects in coelomocytes (PubMed:18923146, PubMed:26783301). RNAi-mediated knockdown results in defective endosome maturation with the accumulation of small vesicles near the gut lumen and large endosomes/lysosomes on the basal side of the cell (PubMed:25273556). Double knockout with either sorf-1 or sorf-2, results in larger endosomes and larger lysosomes and thus suppresses the endosome/lysosome fusion defect in the vps-18 single mutant (PubMed:26783301).