Small Overhangs are elements of a model that are connected along one edge and then extrude unsupported for a short distance.
Why is it Relevant? Small surfaces extending off the main body of a model are susceptible to breakage or failure for a number of reasons. Learn what minimum spec to use when modeling to avoide breakage.
3D printer requirements. Some 3D printers will print supports to ensure the quality and integrity of an overhang is maintained, some 3D printers will always have support material in the build to maintain overhangs, and some printers will be unable to support overhangs unless they are quite thick because there is no support.
Post Processing. Many 3D printed parts will require some sort of handling after they come out of the machine. Very thin protrusions are susceptible to breakage at this point so 3D print Service Bureaus will set a minimum thickness of extrusions or overhangs to reduce breakage costs. If you REALLY need to have thinner parts for a high resolution model, you can look in to having your part printed on a more expensive 3D printer, or contact the service bureau and see if they are willing to try for you if you are willing to cover the cost of broken parts.
Shipping. Similar to post processing, very fine detail sticking out from the body of a model is susceptible to breakage during shipping. The better service bureaus will take steps to ensure your part is packaged specially to avoid breakage during shipping.
What is a Small overhang and what is a Large overhang? This can be arbitrary so developing a theoretical framework can be useful. Is the overhang relatively large or small compared to the support holding up the overhang? A small overhang would be inherently stronger than a freestanding element, it would typically have better sheer strength.
In the architectural example above, the roof is indicated as a small overhang. The unique beam structure above the roof would is a great example of a gray area. The primary difference is how secure the roof is in the model as opposed to the beams. The roof is attached completely along one edge, is a solid "sheet" and supported periodically by posts that are thick enough to provide structure to the model. The Beam structure has small attachment points, and the elements are exposed on 4 sides.
On a character model, an outstretched arm would be a large overhang, while an arm that is bent with a hand resting on a hip would be a small overhang because it is secured on two ends.
In the example on the right, The hand touching the face would be ok as a Small Overhang because most printers will fuse the two to some extent. The other arm would be a problem because the hand is quite large and heavy relative to the thin arm supporting it. If this were to be printed at a small scale I would suggest using the Freestanding Elements minimum thickness. If the model is quite large it may not be an issue for minimum tolerances.
The tail should use the Freestanding Elements tolerances.
The ears are fine as a Small Overhang.
*Eyebrows will not magically float above your model when shipped to you. If the model is small, they will likely get lost in post processing.
On a car model, an open hood could be considered a small overhang if the hood were connected all olong the lower edge (low poly model). If the model were a large scale and high enough resolution that the two hinges were the only attachment points, the hood would likely not survive the printing, processing and shipping process from most 3D printers. It could be printed separately and applied later or the seam could be filled or the hinges could be seriously altered.
With the two example car models above the spoiler paints an obvious example of a large vs. small overhang relative to the support. The Ferrari Spoiler and supports would need to maintain the minimum thickness specified for Freestanding Elements, while the Porche could use the Small Overhangs settings.