In our applications, we use a sans-serif system font stack. This means that we have no single typeface, but rather use the default sans-serif font that is installed on a users device. These are the fonts that are typically rendered per device:
macOS and iOS
SF Pro Display (Font sizes 19px and above)
Our font measurements are based on the line-height of the text, spacing is measured from the edge of the text box.
Main titles, used for things like a page title. Use only once per page or per modal.
Highlight secondary titles on a page, these should be used sparingly.
Deep headings and for highlighting important pieces of information.
Use for the body copy this can we be anything from a couple of words to a paragraph