There are 2 levelling related skills, Required Experience and Experience Gain.
Required experience is how much experience you require for leveling up a job or multiple jobs and experience gain is how much experience do you gain by killing something. Both of these are applied only when you gain experience after killing demon/monster, human etc.
Here is the list of different skill levels and their corresponding effects for each skill.
First is The level of the skill. (This is the required experience). [This is the experience gain]
Level 0 = 1x Exp required 1x Exp Gained
Level 1 = 1/2 Exp required 2x Exp Gained
Level 2 = 1/3 Exp required 3x Exp Gained
Level 3 = 1/5 Exp required 5x Exp Gained
Level 4 = 1/10 Exp required 10x Exp Gained
Level 5 = 1/20 Exp required 20x Exp Gained
Level 6 = 1/40 Exp required 40x Exp Gained
For example: You have Level 4 Required exp active, which is 1/10 exp required and then suppose you need 100 exp to level up once then after using this skill you will only require 10 exp to level up.
For example: You have Level 4 Experience gain active, which is 10x exp gained and then suppose you get 100 exp for killing a monster then after using this skill you will only require 1000 exp from killing the same monster.
Here both the skills stack, so if both the skills are at level 4. Then jobs will level up 100x faster than normal. If one is level 3 and the other is level 4 then 50x faster and so on.
Here the BP adding system is the same as the equipment section, which is double the BP points added to increase a level.
First is The level of rarity. (This is the required BP). [This is the total amount of BP used to reach that level]
Level 1 = (1) [1]
Level 2 = (2) [3]
Level 3 = (4) [7]
Level 4 = (8) [15]
Level 5 = (16) [31]
Level 6 = (32) [63]