The short version
Nine Iron answers one question: which club should I hit from here? You give it a distance, or let GPS measure one, and it shows the club from your own logged shots at this kind of place. Everything else in the app exists to make that answer more accurate.
1. Start with your bag
Choose your clubs and either enter your distances or pick a starting level. Starting numbers are labeled as such, so you always know how much to trust them. You can add a club or change one at any time.
2. Tell it where you are
Nine Iron has three places: the course, the range and the bay. Each club keeps a separate number for each one, because the same swing travels a different distance off a mat, off a range ball or off real turf. Save a spot, and the app switches to the right screen when you arrive there. That part needs location on.
3. Get one big answer
Set the distance and the club is the biggest thing on the screen, in high-contrast colors. There are three color settings: Follow my phone, Daylight and Night.
4. Log what really happened
Save a shot from the Log tab on your phone or from your watch. On the course, you can stand where the ball started, walk to where it stopped and let GPS measure it. A number reads Rough until it has enough shots behind it, then it reads Yours.
On the course
Add a course by name, then set each green the first time you stand on it. The saved point is yours, it works without a data connection, and it costs nothing to keep. A free OpenStreetMap search is there as an extra, though public map servers can be unreliable, so the app doesn't depend on it. Keep score as you go, and turn on Competition mode when you play under rules that limit club advice.
In a bay
Bays have their own numbers. See target distances, use the venue's house clubs as a separate set, keep score in target games, or snap a photo of the bay screen and save the numbers it reads. The picture is read on your phone. Distances are ones seen on venue signs, and venues differ.
Swing review
Film a swing from face-on or down the line, and the app shows where to put the phone. It works out your tempo and how far your head moved, then offers general tips. The video is reviewed on your phone and isn't sent anywhere. These are tips, not coaching, and no result is promised.
On your wrist
The Apple Watch app gives you the club, a quick log and your score. Starting a golf workout keeps GPS running during a round, and the workout is saved to Apple Health on your device.