Most patients are coming in for the same thing, so picking the same diagnosis at every visit gets old fast.
Picking the diagnosis
The superbill page has a Diagnosis (ICD-10) section listing your practice's own code list, with a filter box that matches on code or description. Tick whatever applies to this visit; each tick saves as you make it. You need at least one diagnosis before a superbill can be generated. If a code you need isn't on the list, add it under Settings > Billing, where you can also remove codes you never use. Removing a code there doesn't change a visit that already has it.
Saving a default for the patient
Once codes are ticked, Save as this patient's default stores them on the patient. The button stays disabled until at least one code is ticked, and saving again replaces the earlier default.
On a later visit that has no diagnosis yet, the page shows a banner reading Suggested from this patient's saved default, with the codes and two buttons: Apply and Dismiss. Apply ticks those codes and saves them to the visit. Dismiss hides the banner for that page view. Nothing is filled in until you press Apply.
Without a saved default
If the patient has no saved default, the banner suggests the codes from their most recent other visit that has a diagnosis, labeled as this patient's last visit with a diagnosis set. A saved default always wins over the last visit. The suggestion appears only while the current visit has no codes of its own, so it never overwrites a choice you've made.
Diagnosis codes are separate from the rest of the chart: copying from the last session doesn't carry them over. Everything else about the document is in Superbill export.