There were two exhaustions. Spells and onUse. Most of OTS are using it wrongly because of not implemented onUse exhaustion and these servers are using combat/heal exhaustion which is a totally wrong.
In real:
Using a rune was triggering both onUse and spell exhaust and runes were affected by onUse exhaust and some runes were affected by spell exhaust too.
Casting a spell was triggering only spell exhaust and spells were affected by spell exhaust (you cant cast spells when spell exhausted).
Potions were triggering onUse exhaust and spell exhaust but were unaffected by spell exhaust (unaffected means: you were able to use mana fluids when spell exhausted. But you were unable to cast spell right after mana fluid).
Potions had one sec or less (not sure) onUse exhaust while runes had 1 sec exhaust. Some runes were triggering longer exhaustion, like SD.
You were able to use mana potion second after using SD, but you were unable to cast UH or exura vita because your spell exhaustion from SD.
Some weak offensive spells had 1 second spell exhaustion. Like exori vis, so you were able to spam it.
Probably there was also a exhaustion buffer. When it was higher then one second you were exhausted but when it reaches 0.9 sec you was able to use a spell what was adding 1.0 sec to the buffer (1.9 sec total in buffer now). After that you had to wait till it reach values below one second. Maybe it was a thing or it was just a poor tickrate or poor connection what makes you look like you used two runes almost instantly one after one.
The old system doesn't differ much from the new one. In new system you have spell exhaustion split into offensive, defensive and supportive. Also lots of exhaustions were cut in half, so you can spam potion, exura vita at once. But you are still unable to use potion and SD at once because both are using onUse exhaustion. Also onUse exhaustion in new system is connected with autoattack (not sure if it was in old system too). If you use mana potion every second then your character will stop attacking, but attack doesn't trigger onUse exhaust.
That's all I remember from 2005-2006 when started playing. That's not much info but trust me, OTS system with combat/heal exhaustion was just a failed attempt to copy real system.
