A J-code identifies the drug and defines the billable unit — J9271 is pembrolizumab priced per 1 mg, J1745 is infliximab per 10 mg. The unit is part of the code's meaning, so the quantity on the claim is a multiple of that denominator rather than the dose in milligrams.
Not every clinician-administered drug has one. Newly approved products and drugs with low volume are billed on the miscellaneous codes J3490 and J3590 until CMS assigns a permanent code.
Example
440 mg of a drug coded at 10 mg = 1 unit is billed as 44 units, not 440.