Literacy acts as an interface when understanding or producing language determines what a person can do in a system. Being able to open a document is different from being able to use it.
Illustrative example
Two tenants receive the same written notice. One can identify the deadline and compose an objection; the other can see the date but cannot tell what must be challenged. The notice offers both the same information but different practical choices.
Literacy Is the difference between being able to formulate your own reasoning and simply selecting from pre-packaged options, between understanding a text and merely consuming it.