The deployment server is used to deploy configuration files (like inputs.conf or props and transforms...) / TA's / Apps to end points that have a universal forwarder on them. It allows you to push config changes to all your end points without having to do it manually one by one.
There are some apps that require the python libraries that come with a HF. This is why some apps may require the use of a Heavy Forwarder.
Types of forwarders: http://docs.splunk.com/Documentation/Splunk/7.2.0/Forwarding/Typesofforwarders
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