The materials' total current ATK.

Think about it like this: In Synchro Summoning, if I have "Junk Synchron" and "Speed Warrior", whose Levels are 3 and 2 respectively, I can Synchro Summon a Level 5 Synchro Monster. But if I were to equip "Speed Warrior" with "Synchro Boost", which increases its Level by 1, my option for Synchro Summoning just changed from Level 5 to Level 6, so rather than Summoning "Jun Warrior", I'd have to Synchro Summon "Junk Gardna" in its place.

Similarly, if I have "Lancer Dragonute" and "Twin-headed Behemoth" currently at their original ATK on the field, which is 1500 for each of them, then I can Ignis Summon a 3000 ATK Ignis Monster, like "Volspire Dragon". However, if "Twin-Headed Behemoth" has Special Summoned itself using its effect, its ATK now becomes 1000, and I would have to have a 2500 ATK Ignis Monster in order to perform an Ignis Summon with these two. Or, as an alternative, I could find a way to increase the ATK of "Twin-Headed Behemoth" by 500 to return it to its original 1500 ATK (Perhaps with the aforementioned "Synchro Boost") and then Ignis Summon "Volspire Dragon".

Make sense?