This is a guide to EverQuest spell data and how to read it (courtesy Yther Ore).
Another division, is the Spell Type which is based on the resist check of the spell or song. There are the original five basic resist types, along with two that use the a combination of the original five, and a final that was added with the Serpent's Spine (TSS) expansion, and is only used for events / spells from TSS and beyond.
Original Five Resists are:
Since Spell Type and Counters are often confused, I'm going to explain Counters now, even though they are only limited to debuffs and damage over time (DoT) spells. Counters are ways to determine how much curing, if any, is required to remove an effect. There are three original types of Counters, and one for Serpent's Spine and beyond effects as follows:
A common mistake, is thinking that since almost all Slows use a Disease Counter that they are also Disease Based spells, when if fact, only an extremely rare few are Disease Based and most Slows are Magic Based. Counters are in no way related to Spell Type. More on Counters further down in the Cures section.
A final major division on duration spells is whether the effect is Cumulative or not. Cumulative effects mean, each one that is in a different Spell Slot (see next paragraph) adds to any other. Most normal buffs and debuffs are Cumulative. The exceptions include:
Attack Speed (Haste / Slow)
Another note, is the ranking, is most negative first, then most positive, such that having any amount of slow or snare will be the active buff even when a higher haste or run speed buff is also on. As an example, say a monster is self-buffed with a 60% haste, and you cast a 30% slow and it lands, resulting in the 30% slow being the active buff until it wears off, at which point the mob will be hasted again.
Spell Slots are the mechanisms used by EQ to determine stacking and overwriting / blocking. When you look at the spell data for any single spell, song, or effect, it will list one or more different Spell Slot numbers each followed by a Spell Attribute that is modified, and generally an amount or range of amounts for that attribute. As an example of overwriting, let's look at Skin Like Wood and Valor. As you can see...
Skin Like Wood is shown as:
1: Increase AC by 3 (L1) to 4 (L10)
2: Increase Max Hitpoints by 11 (L1) to 20 (L10) and
Valor is shown as:
1: Increase AC by 12 (L32) to 13 (L40)
2: Increase Max Hitpoints by 164 (L32) to 200 (L50)
3: Increase HP when cast by 164 (L32) to 200 (L50)
In this situation, the best of the slot 1 and 2s will overwrite and/or block the lesser. So if you had Skin Like Wood on, and someone casted Valor on you, it would overwrite the Skin Like Wood. Conversely, if you had Valor on and someone casted Skin Like Wood, it would be blocked, and the caster would get a message that it didn't take hold.
Austerity is shown as:
1: Stacking: Block new spell if slot 3 is effect 'Max Hitpoints' and < 1100
2: Increase Max Hitpoints by 800
3: Increase HP when cast by 800
4: Increase AC by 48
5: Stacking: Overwrite existing spell if slot 3 is effect 'Max Hitpoints' and < 1100
Direwild Skin is shown as:
1: Increase AC by 52
2: Increase Max Hitpoints by 926
3: Increase HP when cast by 926
4: Increase Mana by 10 per tick
5: Stacking: Block new spell if slot 1 is effect 'AC' and < 1173
6: Stacking: Block new spell if slot 2 is effect 'Max Hitpoints' and < 2925
Heartwood Skin is shown as:
1: Increase AC by 70
2: Increase Max Hitpoints by 1381
3: Increase HP when cast by 1381
4: Increase Mana by 17 per tick
5: Stacking: Block new spell if slot 1 is effect 'AC' and < 1235
6: Stacking: Block new spell if slot 2 is effect 'Max Hitpoints' and < 4380
From this, we can see that if you had Austerity on, and someone tried to cast Direwild Skin, that it would be blocked due to the Austerity blocking restriction. If Austerity didn't have the blocking restriction, since Direwild has more HPs than Austerity, it would overwrite it. However, Heartwood Skin will overwrite Austerity, even with Austerity's blocking attribute.
Following this train of thought, one might ask, what happens if the HP attribute was in different Spell Slots for each spell? Very good question, and the answer is, both (or all) the spells will stick. But will they stack? Well that depends on whether the effect is Cumulative or not. Since HPs are Cumulative, then such buffs would stack. This is why most raids go with the Dru Skin + Clr Symbol and AC buffs over the Clr Aego line, as the former gives more HPs and AC, as well as mana regeneration, but requires three buff slots instead of one. Some tanks still go with the Aego line to save buff slots, and have enough room for trash clickies to keep from getting completely dispelled. For details, check out, Heartwood Skin + Symbol of Darianna + Ward of the Devout. Haste, on the other hand is Not Cumulative, so you can have multiple co-existing haste buffs, but only the best of those will actually be in effect. Refer to Miraculous Visions + Celerity + Hastening of Ellowind.
One thing to note about Haste buffs, is there are multiple types and eventhough the same types do not stack with each other, different types will. Also there are two hard limits to haste. The first is normal haste which includes Increase Attack Speed, Increase Haste v2, and Worn Haste which cap at 100% total. The second is overhaste which includes Increase Haste v3 and caps with normal haste at 125% total.
One note of exception on this pattern of sticking / stacking, is that damage over time (DoT) spells with debuffs will stick and the DoT portion will stack, even if the debuff is the same Spell Slot on each spell. Flame Lick and Immolate are examples, and both will land, and the DoT portions will stack, but only the Immolate's AC debuff will be in effect. Strangely enough, even the DoT portion will stack when the same Spell Slot if casted by different players.
Finally, how do you cure an effect. If an effect has a Counter attribute, then you use the corresponding cure to decrease the Counter to zero. If the effect doesn't have any Counters such as the Malo / Malaise line, then only special cure types, that don't work on Counters will work. Such cures include the Radiant Cure AA-line, and the Shm remove detrimental buffs line such as Pure Spirit. Cancel Magic lines also work on these types of effects, but dispel any buff in a somewhat predictable order. Such effects without a Counter are often called Incurable eventhough they mostly can be with the aforementioned tools.
A final note on spell stacking, is that of Haste. There are two types of casted haste, Normal shown as Increase Attack Speed and Overhaste shown as Increase Haste v3 on the Spell Data. There are three types of Haste, worn (no buff icons in the buff windows) and the two casted types. Worn + Normal casted haste has a limit of 100% total. Casted Over-haste will go over the 100% limit, if you are at the limit or close with the combination of worn + normal.
If the description of sticking and stacking is still some what confusing, might look at this post.