Internal threads, in this context, are created by the application itself - so Lock & Monitor could be used allow synchronisation of access to a resource by different threads within a single application.

Internal vs external threadreddit

External threads are created by other applications - so Mutex & Semaphore could be used to allow synchronisation of access to a resource by different applications.

Internal vs external threadchart

Lock or Monitor is one and same it works with internal threads generated by an application. Lock or monitor does not have control over external threads.

From your quote "internal threads generated by an application" From later on in the article "External threads basically coming from outside of an application"

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