Plays an important role in B-cell response to antigen that acts both as a negative or positive coreceptor. Inhibits B-cell receptor (BCR) signaling in the absence of CR2 stimulation but engagement with CR2 and the BCR lead to a superior calcium response compared to CR2 and BCR costimulation (PubMed:30107486). May be involved in B-cell development and differentiation in peripheral lymphoid organs and may be useful markers of B-cell stages. May have an immunoregulatory role in marginal zone B-cells. May play a role in fertilization (By similarity). {ECO:0000250|UniProtKB:Q68SN8, ECO:0000269|PubMed:11453668}. This is the function of FCRL5 (Fc receptor like 5, Ensembl gene identifier ENSG00000143297).