So, I do realize that because I don't go too in depth regarding game mechanics that certain aspects like skill lines are not overly clear.
So here goes:
Skill lines are ranked from Tier 1-?.
Normally, skills start at beginner.
When a skill in a skill line reaches Adept for the first time, the "tier" of the skill line becomes rank 2.
Rank 2 skill lines allow for the learning of higher quality skills.
The skills follow the naming titles as follows.
beginner, adept, expert, master, grandmaster, legendary...
Examples to show how skill lines and skills work.
random mace welder learns the "Smash" skill. they grind and raise the skill to adept.
Mace skill line ranks up to Tier 2.
They chose to learn holy magic to be a paldin.
they learn the holy/mace combo skill holy smite which requires Tier 2 mace skill line and tier 1 holy.
They repeatedly train holy smite to adept.
because Mace is already at tier 2 that does not change, but holy grows to tier 2 allowing for more skills to be able to be learned.
So, spoiler warning:
When Jericho shows the system he is a master pianist and gets that skill registered as "grandmaster" that skill line ranks up to tier 5 allowing for much higher level abilities to be learned.
New skills learned from the system (via teachers, books, etc. The skills start at beginner.
New skills brought from reality into the game are registered at the quality of skill possessed by the person.
There are special skill lines that teach the basic skills of each tier upon the line increasing. Monster skill lines are included in this list.
Hope this helps. if it is still confusing please help me know where so I can clarify it better. Thanks!
Finally was able to get it into the book a bit more properly. It can now be found in chapter 3.