Just like the default UI packs available for iPhone, Android, Web..., etc. Designers can create custom packs and add their own components to it. Custom component packs are available across all projects in that user's account and it also can be shared with other Team Members.
About sharing custom packs:
Custom component packs are automatically shared for all Team Members with TeamPack/Business licenses. For example if you have a 3-user TeamPack license, Any custom component pack created will be available for all 3 users of the TeamPack license.
Steps to create custom packs:
1Open category listing from component sidebar.
2Click on "Create a component pack" icon.
3Enter a name for the custom pack.
4After the new pack is created, it will automatically open in component sidebar or you can also select later from the pack listing just like selecting any other pack.
5First select the components in the editor (it can be any component, image, icon or group of comps).
6Click the 'Add component to Pack' button in the left sidebar.
7Give a component name.
8This adds the component to the pack immediately.