I have added a new article to my own #HIG for Mac apps: Toolbar Guidelines.
This article goes over how to design, organize, and use built-in #NSToolbar & #NSToolbarItem objects and their features.
It also goes over system behaviors when toolbars are mixed with Sidebars, Inspectors, & multi-content layouts!
As a bonus, it also goes over bottom bars & accessory bars. Check it out and let me know what you think!