Based on the translation provided by Jesper, it is possible that there may be an HMI panel connected to the PLC configured to report which page it currently displays to the PLC in N7:21.
If the OP can simultaneously connect the programming software and the HMI, he can try changing to pages 10, 11, and 12 and watch to see if the value in N7:21 changes. also, he could look at the content of those HMI pages and see if they relate to the logic in the associated subroutines.
As Drbitboy and others have pointed out, there are a lot of ways to write a value to a PLC variable, some of them rather obscure. Finding which of the possibilities is active in any particular system can be a challenge.