Farz Ayn. Is an act that is obligatory on every individual such as praying and fasting and he will be sinful if he does not do that. With regard to beneficial worldly knowledge it is a communal obligation or is permissible.
Public users are able to search the site without a subscription. It says in Kashshaaf al-Qinaa 1411. But a fard kifaya can easily move from this state to the state of fard ayn فرض عين an obligation which must be performed by anybody except people who have special conditions.
74 Kudari Usul al-Fiqh p.
A Fard Ayn individual duty is an act that is obligatory for Muslims individually each will be rewarded for performing it or punished for failing to perform it. Fard means Compulsory. They are fards that are obligatory for each Muslim to fulfill. For example if Muslims dont have their own physicians anybody would be asked to learn this science until we would have a capable person if nobody re-reads the sources and does ijtihad we are all asked to do so.