Status resistance and elemental defense are categorically different, despite looking quite similar in the status screen and using a similar calculation.
- Status resistance applies to bleed, poison, toxic, curse, and petrification buildup, but not the HP damage or effect inflicted, if any.
- Elemental defense directly reduces HP damage from fire, magic, lightning, and dark damage types.
As you've observed, statuses only apply an effect when the buildup bar becomes full. You have a separate, regenerating resistance bar for each status. Status "damage" of each type applies towards its own bar, and you suffer the ill effects of the status when you fill up that bar.
Status resistance reduces how much a status-imbued attack of a given strength will fill up that bar. Effectively, having more status resistance gives you a longer bar, and this is how it's reflected in the UI, despite the calculation really being reduced damage rather than a higher cap. 10 points of status resistance is equal to 1% reduction, with a hidden 100 point base bonus. Blocking with a shield reduces status buildup in the same way that it reduces damage taken.
HP damage taken from suffering the poison condition is not reduced by poison resistance; the total is always ~1100. Only poison buildup is subject to poison resistance. This applies to all status ailments: only the buildup is subject to resistance, not the effect: bleed always deals ~200 damage and reduces max stamina by 25%, petrification always kills you, curse always increases your hollowing by one stage.
It may or may not be possible to achieve 1000 effective poison defense, which would make you immune to poison buildup. A DLC adds a piece of head equipment which gives you 900 curse resistance, which makes you effectively immune to curse.
Elemental defense is calculated similarly but directly reduces damage taken: 10 points of elemental defense is equal to 1% damage reduction, with a hidden 100 point base bonus. 1000 elemental defense in a category will render you immune to that type of damage.
Unlike physical defense, elemental defense does not work as a flat damage reduction; you can easily check this by building for 1000 magic defense and asking a friend to cast an optimized Crystal Soul Spear (2000-3000 raw damage) on you.
It is possible to achieve 1000 effective elemental defense in all categories at once by dual-wielding Dragon Teeth and wearing the right armor and rings, though achieving 1000 in one category is much easier. This is quite useful in PvP and in certain areas of the game.