The simplest definition is a plane is a level of existence. Each plane has it own rules and if an entity want to exist in a particular plane the entity must respect the rules of the plane . As humans, we reside in the physical plane at least this is where our consciousness is focused, we are multi-dimensional being, we have part of ourselves in different planes but for our survival in the physical plane, our conscious simply ignore the other existences for example as you read this, you think of yourself as a man composed of blood and flesh or as a soul expressed into an human body. The physical plane is generated by its inhabitant, the humans create their experiences with their emotions, feelings and beliefs. Many thing are specific to the physical plane other plane follow other rules, there is no two planes similar, astral travelers know at least two planes the astral plane and the physical plane, the astral plane is much more like an extension of the physical plane a transition place or doorway to access other planes. Some planes don't have particles, therefore an entity composed of particle cannot existence there unless this entity learn how to transcend itself to make the transition, some planes don't have polarity good/bad above/below, to explore such plane an entity be it a man must transcend itself to the point, he can navigate and understand this plane.There is a long way to go for a simple man to explore the non-polar plane and non-particles plane. This is the object of 'long' studies through many lives. The entity known as Seth channeled by Jane Robert was able to achieve this as stated in the Datre transcript.
In the sleep, we indeed explore other planes in those planes there is a fluidity that allows us to create easily without the restriction of our physical plane. The physical plane was set up for learning purpose no more, no less. Learning is the reason we entered into the physical plane as human.
You are in physical existence to learn and understand that your energy, translated into feelings, thoughts and emotions cause ALL experience. There are no exceptions. - ( Seth - Nature of Personal Reality)
There is an occult tradition that define the astral planes in seven parts with exotic names for each, Robert Bruce the well known astral traveler also identified several different astral planes or kingdoms as he calls them, now, the information you have on astral planes may affect your perception of it, it is best to go and explore and have your own perception of what an astral plane can be. Now, there is no need of astral travel to create a plane a level of existence , artists do create plane through the from of art (Seth). You project your thoughts, feelings and expectations outwards, then you perceive them as the outside reality.