I will try to interpret the given data as I’m not flying Boeings anymore.
In this menu you can update (or move to if you prefer) your present FMC position to one of the calculated positions by
1) IRS
2) GPS
3 Radio nav calculated position.
As this is an action which can have significant implications, a dual action is required :
1) Arm the function
2) Chose which source you prefer by pressing “now”
All sources may not be available and all possible cases are depicted
1) IRS Position is not available (“now” is missing) because the ANP is out of limits : 4.0 while RNP is 0.5
2) GPS position update is not available (“Now” is missing), as it seems that is already the source providing the data (FMC coordinates perfectly match to GPS coordinates).
3) Nav aid position is available and within RNP limits thus “now” is displayed.
Af far as I understand, with this menu, you can intervene to the automatic priority (if GPS not available go to Navaid. If Navaid not available go to IRS), and this action will be permanent(until you change again the source to GPS for example).
Now a possible scenario to use this menu:
You are aware by NOTAM that you will cross an area, in which you may receive incorrect GPS information.
In this case you deselect GPS as the primary source of FMC position and you select the Navaid position.
When out of the area, you can switch back to GPS position.