The term “Language” is used to define, in a simple manner, a mode of communication – it need not be spoken/verbal/sound means. Each language a character knows will have a [Skill] rating of their capability in using it. Language use follows the Levels of Knowledge in terms of how much each level is employed by speakers. A Native Speaker is part of the society the character was predominantly raised in, or assimilated into. Not only is the language itself understood, but the social and cultural references and context.
Literacy/Written/Recorded Communication: This reflects how well communication can be achieved when recorded – it is usually the same as the “Spoken” version of a language unless it draws heavily on symbology and conceptual expression over strictly phonetic. Not all languages have a literary accompaniment. Many literary elements are derived from other regional influences or even wholesale adopted from other languages if it was not evolved with the physical expression.
Understanding Records: For languages, it is the spoken sophistication they are capable of, for a literary work, it is, essentially, a “reading level”; A character capable of speaking at one level often writes as a lower level to be more approachable to a wider audience. A character accessing a body of knowledge must have a practice level of understanding equal to the reading level it is recorded in for it to be fully comprehended.