From FreeCAD Documentation
- Blue: a length dimension in the side view of the hole needs a "⌀" prefixed to distinguish this from a rectangular hole.
- Insert '⌀' Prefix is an easy way to do this, but the DataFormat Spec property can be edited manually as well.
- Green: a simple length dimension.
- It needs some auxiliary geometry (cosmetic points) as it cannot be applied to circles directly.
- Red: a diameter dimension. In case you look along the hole axis and can see the circular shape of the hole the "⌀" may be omitted. To remove it edit the DataFormat Spec property manually.