Skilltrees are the way to specify when which pet will unlock/upgrade which abilities.
You can think of skilltrees like classes for players (for example knight and archer).
This is a skilltree file. Every skilltree has it's own file.
Any Ability has it's own custom option that can be set in the skilltrees.
An upgrade will change the settings of a skill. It can add a row to the inventory or add more damage. Every upgrade is applied on a level rule that determines when it will applied. An upgrade can be applied on fixed levels or on repeating schedules like every 3 level.
Skilltrees are capable of inheritance which means if skilltree
a inherits from skilltree
a will have all the upgrades form skilltree
b and in addition to it's own upgrades.