If by Inactive you mean there is a "dotted line" instead of a straight line between the dependencies it means there has been a connection in the last 72 hours (default Smartscape timeframe), but not on the last 2 hours.
If you want to understand the logic behind that here it is, OneAgents that are installed across all the hosts in environment, they will send the mapping of the entities that are present in each hosts to Dynatrace AI and then our Davis takes all the data and consolidates them and show it as a visual representation to the user as logical constructs that we call it as Smartscape. Connectivity between entities are determined by the communication of OneAgent to Dynatrace server as it reports all the data in almost real-time fashion, AI takes that data and if the communication is not there between entities it will represent the communication in dashed line. If communication is there then a continuous line.
Process groups group together a set of processes that run on across a multitude set of hosts (as long as they are part of the same Host Group) In doing so it allows a user to see the communication flow, scope and footprint of that process group, rather then looking at 1 host, 1 process at a time.
I have had many times where a Senior Leader came to us and asked to map out their Application that runs on a set of hosts. "I want to see the layout for my Virus Scan process on all of our Prod infrastructure" Its simple enough with Dynatrace - Target the Process Group, view it in smartscape. Remove Inactive connections removes out the whole environment and only shows active or recently active connections within the last 72 hours. Which really allows you to showcase how and what that process group talks to as a whole and what it doesn't talk to within your environment.