Events Subscription
A standard Ethereum node like Geth supports subscribing to specific events. Additionally, there are some Ethereum nodes that provide additional custom subscriptions. As you can find in Supported Subscriptions guide, web3.js enables you to subscribe to the standard events out of the box. And it also provides you with the capability to subscribe to custom subscriptions as you can find in the Custom Subscriptions guide.
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If you are the developer who provides custom subscriptions to users. We encourage you to develop a web3.js Plugin after you go through the Custom Subscription section below. You can find how to develop a plugin at web3.js Plugin Developer Guide