This code system http://hl7.org/fhir/modifiers defines the following codes:

CodeDisplayDefinition
aRepair of prior service or installationRepair of prior service or installation.
bTemporary service or installationTemporary service or installation.
cTMJ treatmentTreatment associated with TMJ.
eImplant or associated with an implantImplant or associated with an implant.
roohRush or Outside of office hoursA Rush service or service performed outside of normal office hours.
xNoneNone.