A block library is a collection of blocks that you can use to create instances of those blocks in a Simulink® model. You can create instances of blocks from installed Simulink libraries, and you can create custom libraries to create and maintain instances of your own blocks in models.
You can access the installed libraries from the Simulink Library Browser. You cannot modify the installed libraries. Instead, if you want others to be able to create customized blocks, you can create your own library of your blocks and add it to the Library Browser. Otherwise, you can make your library accessible to the models that use it.
Creating your own libraries is a useful componentization technique for:
Providing frequently used, and seldom changed, modeling utilities
Reusing components in a model or in multiple models
When you add masked blocks, subsystems, or charts from a custom library to a model, the block you add becomes a linked block. A linked block connects to the library block by way of a library link. The library block is the prototype block, and the linked block in the model is an instance of the library block.
The linked block looks and acts like the library block. However, if you change the library block, you need to update the link on the instances. Making changes to an instance can also require additional steps. To learn about how linked blocks work, see Linked Blocks.