OpenGL commands - sequential or parallel

Question

I'm reading this document and I have a question about this sentence:

While OpenGL explicitly requires that commands are completed in order, that does not mean that two (or more) commands cannot be concurrently executing. As such, it is possible for shader invocations from one command to be exeucting in tandem with shader invocations from other commands.

Does this mean that, for example, when I issue two consecutive glDrawArrays calls it is possible that the second call is processed immediately before the first one has finished?

My first idea was that the OpenGL calls merely map to internal commands of the gpu and that the OpenGL call returns immediately without those commands completed, thus enabling the second OpenGL call to issue its own internal commands. The internal commands created by the OpenGL calls can then be parallelized.

Answer 1

What is says is, that the exact order in which the commands are executed and any concurrency is left to the judgement of the implementation with the only constraint being that the final result must look exactly as if all the commands would have been executed one after another in the very order they were called by the client program.

EDIT: Certain OpenGL calls cause an implicit or explicit synchronization. Reading back pixels for example or waiting for a synchronization event.