In queueing theory, a discipline within the mathematical theory of probability, an M/G/k queue is a queue model where arrivals are Markovian (modulated by a Poisson process), service times have a General distribution and there are k servers.
A M/G/k queue is a model where arrivals are Poisson process, service times have a General distribution and there is one server and a buffer of size k-1. If the service time is determined for the exponential distribution the number of entities present in the system in any moment can be modeled like a Markov Chain in continuous time.
You can also refer to my book Queueing Models, Second Edition, 2014, at https://www.novapublishers.com/catalog/product_info.php?products_id=42055&osCsid=989f8340426607a10330f93e06f7b42d