Some prescription sigs include "TAT" for "'till all taken". This is a useful reminder for the pharmacist to relay for e.g. antibiotic prescriptions. FHIR MedicationOrder.dosageInstruction includes "asNeeded" (code or boolean). Typically pain medication would be taken as needed while heart medication would be taken rigorously. Does marking "asNeeded" false imply that the prescription must be finished or should "TAT" be represented as an extension, e.g. an "untilResolved" extension with a value of false?
I would expect TAT, "with meals", "on an empty stomach" and other similar guidance would be expressed using dosageInstruction.additionalInstructions. (Feel free to submit a change request encouraging the work group to define a starter set of codes that includes these concepts.) "as needed" is used for PRN orders. It indicates that there's some sort of pre-condition that guides consumption. For example "as needed for pain" or "before intercourse". It explicitly means the patient won't necessarily be taking the medication on a regular schedule.