Generally speaking feature services are optional system components that are not needed in all deployments or on all nodes of a cluster. An easy example is the actual CallManager service: in clusters of more than two nodes this service is only run on call processing subscribers, not the publisher, TFTP, or IPVMSA, etc nodes. To avoid throwing errors only the needed feature services are activated (i.e. allowed to start).
Network services are non-optional services for that product: they are required in all deployment scenarios. You have no ability to prevent them from starting but are able to stop/start/restart them.
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