Modifier 24 is appended to an Evaluation and Management service by the same physician during a post op period.  See the CPT® book for the complete definition.

This modifier may only be used with E/M services. 

When a physician bills for a surgical procedure, the post op care for that procedure is included in the global payment.  Surgical procedures have 0, 10, or 90 days.  That means, all of the care provided during that period is paid for in the global payment, and post op visits may not be billed separately. 

However, if the physician sees the patient for a problem unrelated to the surgery, that service is payable.  Indicate that the service was unrelated to the surgery using modifier 24.  For example, an Orthopedist may perform knee surgery on a patient, with a 90 day global period.  On day 55 of the post op period, the patient comes in to the office complaining of numbness and tingling in his right hand.  This visit is unrelated to the surgery.  Submit the E/M service with a 24 modifier. Be sure that  the diagnosis reflects  that the visit was not a post op service.

Without the 24 modifier, the claim will be denied as being included in the global package.

A surgeon may only bill for critical care during the post op period when the critical care is not related to the reason for surgery.  In general, a diagnosis code in teh range of 800.0 through 959.9 (except 930-939) are acceptable. These are trauma related.   Non-trauma related critical care, unrelated to the surgery is separately payable, but be prepared to send the notes.  The question of "related" and "unrelated" are not always crystal clear.