Best practices
Support the Action button with a set of your app’s essential functions.
For each action you support, write a short label that succinctly describes it.
Prefer letting the system show people how to use the Action button with your app.
Platform considerations
iOS
Let people use your actions without leaving their current context.
watchOS
Consider offering a secondary function that supports or advances the primary action people choose.
Prefer using subsequent button presses to support additional functionality rather than to stop or conclude a function.
Pause the current function when people press the Action button and side button together.
For the complete guidance, including worked examples and illustrations, see the canonical page: https://developer.apple.com/design/human-interface-guidelines/action-button