Learning relies on knowledge, and knowledge has a scale of multiple levels of capability within the applicable reference body and/or practice it references. The levels measure the quantity and scope of information they impart and what the potential operational capabilities are if they are used.
Measure (Quality) of Understanding/Grasp of Information
“ Requires Study“: If some reference work or communicated knowledge “requires” formal study, it may also indicate at what level the studier must have with the communication mechanism/language the communication is being done in.
Uncommon: ‘Fluent‘ [d20] Typical target is DC 15; proficiency bonus or Lore bonus +4 Considered part of Common Knowledge
Scarce: [d20] Typical target is DC 20; proficiency bonus or Lore bonus +6 Considered part of Uncommon Knowledge
Rare: “Humans walk upright on 2 legs, have babies, and as mammals have live births after 9 months gestation, typically raising them as part of a multi-person family or extended social unit. Physical characteristics are based on DNA inheritance, which are typically manipulated in modern society using at-home kits and/or doctor-patient gene-therapy sessions. Parents can be of any gender given modern breeding science, and offspring are designed to be disease and age resistant in addition to physically tailored to adapt better to their target planetary environment.” [d20] Typical target is DC 25; proficiency bonus or Lore bonus +8 Considered part of Rare Knowledge
Searching for Information: Combining bodies to search and discover information allows the searcher to use the highest of the levels. If more than 1 is used, each additional source/library of Knowledgeable or better grants +1 on total level used for search. Without technological assistance a character can cross reference no more libraries at once than their Intelligence bonus. In addition to the normal bodies of knowledge, there usually exists Forbidden/Hidden Knowledge which is kept from most seekers.