I don't know what the meaning of MASTERNODE's coexistence with POS is? POW+MASTERNODE has been proved to be a more stable choice
While making it simple and short
Masternode is a server on a decentralized network. It is utilized to complete unique functions in ways ordinary nodes can’t. It can be used for features like direct send, instant transactions , Anonymous Transaction or private transactions.Now Lets go long, For easy understanding leave master in one side and lets go with NODES,
A physical network node is an active electronic device that is attached to a network, and is capable of creating, receiving, or transmitting information over a communications channel, according to that definition a node means one computer (Assuming your computer with Masternode Coins) in a network that is communicating with other computers (Other Masternode Computer) in that same network. Node strongly suggest the truth or existence of your computer is part of a specialized computer network. Now lets go with
Masternode Masternode will need to lock a specific amount of coins (The coin you want to Host Masternode For) in a wallet address which can't be spent, which will only be used to operate the masternode, If funds are removed, Spent or Transfered to another wallet than hosted masternode wallet the user (Your Computer or Node or Masternode) is no longer eligible to receive incentives generated by providing masternode services to the ecosystem because you don't have collateral coins which needed to be on masternode to stabilize the network.
Nodes A node is a point of intersection or connection within a network. In an environment where all devices are accessible through the network, these devices are all considered nodes. The concept of nodes works on several levels, Internet nodes are sometime and somewhere also referred to as Internet hubs. Everyone who supports bitcoin or any other Altcoins should run a full network node. which means storing over 200GB of data (Current Bitcoin Data) it will differ according to coins, and that Data will continue to increase significantly every Second/day/month/year – which is a major drawback. An old computer, or a cheaper device with external storage for example, is sufficient to operate a full network node. A lot of people use VPS servers for this type of purpose as well, even though that does not necessarily work at all times.