Hi Imran,
IF_BOL_BO_COL is the most generic one. It can hold any instance implementing interface IF_BOL_BO_PROPERTY_ACCESS (like model entities, but also value nodes, query objects, etc.).
IF_BOL_ENTITY_COL is meant to store entities only (i.e. variable of type REF TO CL_CRM_BOL_ENTITY).
Regards,
Nicolas Busson.