I want to know what's the difference between the textures Null and Clip, personally i like Null, i use it for every face of a solid/non-solid that the player doesn't see and to block off people from going to certain areas
Null = for optimization map. If you don't want to render some faces of your brush just put null texture and it will be good for r_speed. Null is mainly used in places where player cannot reach otherwise player will see "bugs"
Clip = this texture is for creating invisible walls. With this texture you can block everything everywhere to not let player jump of the map or go to place where you don't want player to be. Player can shot / throw grenade thought clip wall, but he cannot pass. Clip texture is also for making smooth transitions when player walking on stairs or other terrain. When using clip texture on your map make sure the entire brush is textured by clip not only 1 or 2 faces. However don't make brush with clip texture to ent like func_wall.