What do the icons in the Project tool window mean?

Question

What do the icons in the Project tool window mean?

Answer

Identify an icon in this order.

1. Hover the item. The tooltip names the item type.
2. Read the icon as two parts. The base shape gives the item type, such as a class or a directory. A badge or overlay added to it marks scope or a modifier, as the lock overlays do for public, protected, and private class members.
3. Look up the icon on the Icon reference page, which groups icons by UI area: main toolbar, tool window bar, status bar, file types, gutter, bookmarks, breakpoints, change markers, code coverage, and database objects.
4. Browse the IntelliJ Platform icon list by category and match the image. Each identifier, such as projectDirectory or groupByPackage, indicates what the icon is used for. Text search works poorly when the icon's name is unknown, so scan the category for the UI area instead.
5. Disable third-party plugins and reopen the project. An icon that disappears was added by a plugin, and that plugin's own documentation describes it.
6. If the icon is still unidentified, contact support with a screenshot showing the icon in context, plus the IDE version and build number.

Where individual icon groups are documented