Please also see the original source at https://github.com/7elix/TYPO3.CMS.Flat/issues/6 (source)

There are three major types of ICONS in the TYPO3 backend:

1. Module / App icons
2. Record icons
3. Action icons

These icon groups have different purposes:

Identity¶

App icons¶

Task: Click on icon starts backend module view right away. The complete interface and focus changes for the user.
Position: Module menu / "launcher"
Action: Start different App view

Record icons¶

Task: Identify single record and are unique per data type. Reside mostly in tables..
Position: In front of record title / within table always the first column
Action: Opens Clickmenu for list of action

Action icons¶

Task: Trigger primary action directly.
Position: In data tables always within the last column.
Action: Single click starts action right aways - something happens

Visual distinction¶

App icons¶

Colorful background. Monochrome/inverted pictogram in the foreground. Rounded corners. Large dimensions of 32x32 Pixels (high-dpi at 64x64px)
Technology: Rendered in moduleMenu as images by ViewHelper

Record icons¶

transparent background + colorful foreground, No border no corner no background color necessary. Optimized to sit in table list rows at 16x16 Pixels
Technology: Output by IconUtility and img-Tag

Action icons¶

transparent background + and monochrome, vectorized (FontAwesome). no border, no corner. Output ~16x16 Pixel
Technology: Output based on FontAwesome / monochrome vector font