Raphael Bousso came up with a modification that the spatial boundary should not be a trapped surface. This led him to come up with Bousso's holographic bound, also known as the covariant entropy bound. Basically, a spatial boundary is valid if, when one considers both null sheets orthogonal to its tangent space, the expansion factors both point in the same direction. This defines inside and outside. The entropy of the interior region is bounded by the surface area of the boundary.