Radiant Zone effect timing

[Bookkeeper]

If I apply the Radiant Zone effect to my quantum field, the mechanics of the Zone trait seem to imply that the field stays radiant until it closes. If so, when do creatures within it actually make saves? It could theoretically be any number of the following:

a) On using the action Radiant Zone
b) On starting their turn within the Radiant Zone
c) On moving into the Radiant Zone [?for the first time during their turn?]
d) On being moved into the Radiant Zone

Creatures don't ever become immune to the effect, so they could theoretically be making multiple saves per turn.

[Xenocrat]

I agree this needs more explanation.

I also need Zone Overlap feat to explain if "Radiant Zone forces creatures to attempt another saving throw" means they roll twice and take worst (misfortune effect), or if there's some implicity/missing immunity that this is bypassing (once?).

[Ryuujin-sama]

Kind of wish doubling up on Danger Zone with Zone Overlap would just double the number of damage dice, instead of just adding 2d6.