They are basically small rubber packings that help enlarge the diameter of IEMs' stem, so that more options become available for users when it comes down to ear sleeve choices. Such a brilliant idea! How come nobody has ever thought of this? You can fit almost any type of ear sleeves to IEMs which have a thin stem.
So I did a quick analysis on various ear sleeves I have, along with the sleeves Inks sent me as well:
THINGS TO CONSIDER: Data of Grado's ear sleeve and Shure EA510S are coincidental. Monster supertip is measured utilizing a proprietary ear canal extension- which means EURI has been fully dissected in order to measure the sleeve- since it does not fit EURI's ear canal!
On silicone sleeves: Due to the lack of bore-induced resonant effect, most of the sleeves has a high frequency attenuation compared to the stock ER38-18.
On foam sleeves: Although Shure orange EAORF2-10S has a low frequency attenuation(possibly caused by acoustic leak) it yields the most linear response.
ON SECOND THOUGHT: I also received a pair of bi-flange Klipsch oval ear sleeves. Here's the measurement: