Everything is defined by its opposite. If it has no opposite, it therefore is everything. Anything and its opposite make up everything. This applies only within its relam.
For example. Everything in the realm of temperature is either hot or cold (draw the hot/cold line wherever you'd like). The definition of hot is "that which is not cold". In fact, where cold starts, is where the boundary of hot exists. Similarly in ideas, all ideas within the realm of logic are either true or false. But that is only true if the statement is logical. If the statement is illogical, it is neither true nor false. An example would be the statement, "this statement is not true". The statement is illogical, and thus, neither true nor false. Though I've read people talking about meta-statements, but there is no need for that here, and would simply be overkill.
Realms themselves are defined by their opposites. The realm of temperature is defined by that which has no temperature. That is seemingly akin to the realm of physicality. If a relam has no opposite, it is all-inclusive.
Going a bit more esoteric. The other way to define something is to understand it. Then one automatically knows where the boundaries are. If that cannot be understood, the boundaries are searched for, which give the person a feeling for what it is. And this latter method, searching for boundaries, is used more often, simply because the other method is not always possible.