The Subclass system is one of the unique features of Lineage II and that is what won me over so many years ago. This feature would allow you to add up to 3 different classes to your main character with a max level of 80 on each, of course certain restrictions on race and class would apply and some of them still apply to this day, like not being able to have a Dominator as a subclass/dual or a Warsmith. Of course back in the day there were more restrictions regarding both class and race for example, no Light Elf could have any of the Dark Elf classes and vice versa.
This changed at some point and you can only have a single subclass to your character, once your character hits 105, you can either click on the notification message to add a subclass or you can visit the NPCs at Aden and add the subclass of your choise.
Joachim will help you add a sublcass or reset it, if you want to. There are certain restrictions for classes that are race/gender locked. You cannot have as a dualclass a Death Knight, a Dominator, a Maestro, a Sayha Seer or an Eviscerator. In terms of resetting, as of the November 2023 update, keeping your level when reseting a dualclass is a game feature since up until this point keeping your level was only possible during Red Libra Events.
The benefits from having a high level dualclass is passive skills and a collection, there are 2 categories of skills, the ones from 85 to 105, and 107 to 110. There are 7 certificates for levels 85, 90, 95, 99, 101, 103 and 105 and 3 more for 107, 109 and 110. You can only use each group of certificates for its respective category, so a 85-105 cannot be used to acquire a skill from the 107-110 and vice versa.
Gallias will allow you to get certificates based on the levels of your dual class, learn the skills or forget them (reset). You get the certificates while being on the dual class and you learn/reset them, when on main class.
You cannot see the list of skills if you do not have at least 1 certificate in your inventory.
These are the 2 skill categories that you can choose skills from, there are 7 certificates for 105 skills and 3 for 110 skills.
Attack, Defense, Speed ( attack & cast speed), Condition can have up to 4 levels each, 1 certificate per level, Vampiric, Physical, Magical and 2 certificates each, the same goes for Vital, Recovery and Death, finally the proc skills Berserkers Rage, Masters Rage and Light of Protection require 3 certificates. Now the 110 skills require 1 certificate per skill.
Using the skills now is something that depends on your class, what your goals are and at what point in the game you are, if that is purely PvE or you are more PvP oriented.