One balance, most things
Credit on an Apple account covers apps and in-app purchases, storage plans, films, books, and subscriptions billed through the account — including services whose own gift cards would not work for a subscriber billed this way.
For many recipients the quietly most useful part is the storage plan, which is a small recurring charge almost everybody pays and nobody enjoys.
Who bills them
The deciding question throughout this site. If a recipient subscribed to a service on an iPhone or iPad, Apple bills them and this is the credit that reaches it. If they subscribed on the web or on Android, it is not.
Have them look at their subscriptions list. It names what is billed through the account.
Region
Apple accounts are country-specific and credit is too. The account country cannot normally be changed while a balance remains, which makes a mismatched code an expensive mistake rather than an inconvenience.
Ask which country's store they see.
Family sharing
Purchases can be shared across a family; credit cannot. Balance belongs to the account that redeemed it. A code given to a child on a family plan pays for that child's purchases, not the organiser's subscriptions.
Where the organiser pays for a family subscription, credit has to go to the organiser.
Amount
Anything works — the catalogue is wide enough that nothing is stranded. Round numbers read better on a card.
Asking for one
Country and amount, plus who bills them if the gift is aimed at a subscription. See Apple Music and the software guide.