what is the diffetence between screening and blocking in the game of volleyball
Answers
The official rules (as you cite in your question) prohibit screening. Unfortunately the rules themselves do not give an exact definition that you could use. In referee trainings, however, you will get more info on how to judge screening.
Everything that helps the serving team to prepare for defense is allowed. So each player of that team can take any position on the field (within the constraints of the rotation rule) which "makes sense". Therefore the front players are allowed to group near the attacker(s) and raise their arms as preparation for blocking. Also the back row players are allowed to take reasonable positions.
What is definitely not allowed is to move during your own player serves in order to block the sight. (It is not allowed to leave a line of sight between reception player and serving player and then "closing" it shorty before the service is hit.)
It is also not allowed to take "unreasonable" positions in the field. I.e. a back row player is not allowed to stand directly in front of the serving player, if he normally would take another position.