IOUserClient::connectClient is an obscure IOKit method which according to the docs is supposed to "Inform a connection of a second connection." In fact IOKit provides no default implementation and only a handful of userclients actually implement it, and it's pretty much up to them to define the semantics of what "informing the connection of a second connection" actually means. One of the userclients which implements connectClient is IOAccelContext2 which is the parent of the IGAccelContext userclient family (which are the intel GPU accelerator userclients.) IOUserClient::connectClient is exposed to userspace as IOConnectAddClient.
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