by soluuloi (from Official Forum)
The proof can be made by merging 3 parts. You will get 1 random part per week. Some missions will give you a whole proof or some parts of it. These parts can also be found in the final spots of a lot of maps. You can also steal a part if you successfully raid someone (that person must have some parts, if he has no part, there is nothing for you to steal). It also means that, your parts can be stolen by other players. So, if you have enough parts to create a proof, do it immediately.
Now, you will want to use your precious proof on someone. But who? Of course on someone who has the potential. 2nd job has higher stat growth than 1st job, so the character you are going to use it on must has a low lvl for more stat gain. Also, character with no bloodlimit, most of the times, has lower stats gain than a character that has bloodlimit. So, you will want to use your proof on a low lvl character that has bloodlimit. But it is not always true.
She is a hero that has DoubleAttack skill at lvl4, she has bloodlimit, she has low lvl. Should I use a proof on her right now? No! Because she lacks of skill. Character will get the first skill at lvl10. In this case, if she changes her job right now, she will lose the chance to learn Shield skill. There are 2 outcomes:
If she becames an Axe Master. She will has only 1 skill till the end of her life. Because Axe master's skill is also DoubleAttack, she cant learn it anymore, lvl4 is the maximum of the skill. Like this poor guy who has only 1 skill at lvl42 I changed his job at lvl1 :
If she becames an Axe Guard. She will be able to learn CounterCanceler which is sadly, an inferior skill compares with Shield skill. Shield = Protect from melee attack + Protect from counter attack + Protect from arrow. It triggers whenever you are going to be hit by a physical attack (not only melee).
So, I will use the proof on her when she is lvl10. At that time, she will have DoubleAttack lvl4 and Shield lvl1. Because she will still able to to improve Shield lvl no matter if she becomes Axe Guard or Axe Master, she will has DoubleAttack lvl4 and Shield lvl4 very soon! Skill is more important than stat. Weapon, armor, accessory can fill the lack of stat but nothing can fill the lack of skill.
The above information only really applies for people who have brand new characters. This is because any children born from different job parents will always have 2 skills. So you will almost always want to job-change a character at Level 1. However, keep in mind that the stat bonuses you gain from advanced jobs is % based. So if you have a poor, weak character, he will still be a poor, weak character with adv. job. Once you are confident of having enough proofs to job change all the kids you want, and are able to get more by the time your next generation of children are ready, then you can use proofs however you like. A good rule of thumb in terms of importance:
Starting Stats > Job >= Bloodline