It is simple. Any recurring posture that causes pressure and tension leads to poor posture. It is hard to cope with postural challenges in this case. In simple terms, anything that makes it harder to retain a comfortable posture is a postural challenge. One of the common postural challenges that we all face is the bad usage of tools.
Example includes having your knees under the chair for a long time while sitting for hours.
Having said this, how much does the poor posture affect the body is an entirely different topic in itself.