These are basic qualities of surface. Considering these early on can help you avoid treating spaces as defined only by walls. Opaque surfaces separately two sides completely and fully. The result is physical and visual separation. Transparent surfaces allow full visual connection while creating physical separation. Translucent surfaces create partial visual separation with either full or partial physical separation. A frosted or sand-blasted glass wall allows light and some visual clues to communicate while preventing physical connection. A mesh or other type of screen allows slightly more visual information while allow facilitating sound and air movement.
Relationships between two spaces can be made complex when you begin to combine these three qualities in a single space or single surface.