The eyepatch is a unique feature of some countryballs' decors. Many countryballs with special insignias on their flags use them. They are based off of real eyepatches. In Polandball, eyepatches are used when an important insignia on a countryball's flag interferes with where the eyes on a countryball should typically go. The eyepatch permits the insignia to go overtop of the eye.
One of the most common and well-known eyepatches for countryballs is that of Serbiaball. In some Polandball communities, it is obligatory that Serbia has one. It is not obligatory in all of Polandball, however, for any countryball that could use an eyepatch to use one.
Using an emblem in a countryball's canton or the center of a flag as an eyepatch is uncommon and not typically recommended by communities. Eyepatches are most common for flags that contain their emblems in the hoist of the flag.
However, this is pretty not used so much and is mainly used for 
Serbiaball, 
Austria-Hungaryball (double eyepatch), 
Spainball, 
Portugalball 
Sloveniaball, 
Slovakiaball, and 
Skopjeball.