The difference is based on what version of CPS and Immunization form you use with centricity.
ORU or observations: This was used in CPS10, CPS11, EMR 9.5 and is the only way to record immunizations in those versions. Each component of the immunization is stored individually in separate obs terms.
CL or Clinical List: This is the new immunization table introduced in CPS 12 and EMR 9.8. Most customers have to run a immunization conversion to convert the old records from obs terms to the immmunization table. Also the form used for recording them has to be updated to record the new immunizations to the Immunization table in the database.
ORU uses Linklogic Observation export and CL uses the new Common Event Model.
More information can be found in the help menu in QIE in the "QIE IIS Registry Guide".