Access to Knowledge Bases (KB) is controlled via User Criteria. On the Knowledge Base definition record, these criteria are attached via Related Lists (often appearing as tabs at the bottom of the form).
Can Read: This Related List defines which users, groups, roles, or companies are allowed to view articles within this specific Knowledge Base.
Can Contribute: This Related List defines who can create or modify articles.
"Available to" is terminology used in the Service Catalog, not Knowledge Management. "Can Access" and "Accessible to" are not standard Related List names for KB security.