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Determinate , for a task with a well-defined duration, such as a file conversion
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Indeterminate , for unquantifiable tasks, such as loading or synchronizing complex data
Best practices
When possible, use a determinate progress indicator.
Be as accurate as possible when reporting advancement in a determinate progress indicator.
Keep progress indicators moving so people know something is continuing to happen.
When possible, switch a progress bar from indeterminate to determinate.
Don’t switch from the circular style to the bar style.
If it’s helpful, display a description that provides additional context for the task.
Display a progress indicator in a consistent location.
When it’s feasible, let people halt processing.
Let people know when halting a process has a negative consequence.
Platform considerations
iOS, iPadOS
Refresh content controls
Perform automatic content updates.
Supply a short title only if it adds value.
macOS
Prefer an activity indicator (spinner) to communicate the status of a background operation or when space is constrained.
Avoid labeling a spinning progress indicator.
For the complete guidance, including worked examples and illustrations, see the canonical page: https://developer.apple.com/design/human-interface-guidelines/progress-indicators