Reflex save is your universal save and is in place vs all reflex saves. Save vs fire will be used on effects that are tagged as "BUURN!" in Funky's toolset, if and only if, your base reflex (or perhaps in some odd occasion will/fort) + bonus save vs fire exceeds base + improved universal save. Universal save is capped to 20 and specific save can go up to 25 or 30, cant recall which was the cap. But this requires your gear to have say 3x added save vs fire, 1 for +10, 1 for +8 and last one for +7, netting you a total of +25 vs fire. Having +20 universal and +5vs fire wont give you any extra protection vs fire.
I think specific increased ref/will/fort can go to +25 and specific save, such as vs fire/death can go +30, but someone who has read notes in past 1½ years could confirm that.
Edit: To be more clear with this:
You're lvl 40 rogue, base reflex save = 12 from 1-20 and 10 from 21-40 totaling 22 + 13 ability mod from 36 dexterity = 35 base reflex.
Then your gear gives you +20 improved universal saves. You'll make reflex check of 55 +d20 vs DFB for half damage.
Then you find gear pieces that adds tons of bonus reflex save. Say you find +50 bonus reflex save on your items. The same roll would now be 60 (35 base + 25 (reflex cap)) +d20 vs DFB for half damage.
Then you find these uber loots that have some junk added on green such as added safe vs fire +15 on several item pieces.
This time the DFB check for half damage will be made as base 35 + 30 (bonus save vs fire) for total of 65 +d20 vs DFB.
Hope that clears it a bit