Realm border is the edge of a Realm Bubble. It can either be border between two realms, which is stronger, or between realm and universe, which is weaker. With enough power it is possible to cross this border. For the inhabitants of all the realms the realm border looks like semi-solid barrier that appears to be from frosted glass with blurry glimps of what is behind it.
It has a slight static charge and this can attract certain things like clouds or dust particles.