There are two key words when working with Skype for Business server 2015 that you should be aware  of those are Active and Concurrent. These particular words will come up quite a bit when you are working with a design, capacity planning, or troubleshooting an issue. At the end of the day, regardless  of the scenario you are working with these words are important to understand what they mean and how to use them accordingly.  

What is considered Active?

Active can be considered what is happening at an exact moment in time; such as "How many active Peer-to-Peer or Skype to PSTN calls are taking place right now?"   This would mean calls that are actually taking place at that exact moment.   Another scenario is "How many active users of Enterprise voice are enabled for your environment?"   This could be confusing because depending on what sort of results the person who is asking the question is looking for the question could be interpreted in a few ways.

The last question could be interpreted as the following:

How many people are enabled for Enterprise voice?
How many people are actively using Enterprise voice in the environment?

This leads me to the usage report below.  

The report is talking about how many people performed an action of one of the key modalities who made an IM, Conference, or Call.   There is a difference between being logged on and not doing anything and logged in and performed an action.  You can almost say it depends on the question that is being asked again as I mentioned earlier.    

Figure 1 displays the following information:

Total Logons – Total logons (rather the users did anything or not once logged in) internal and external to the environment.   This could mean that a user signed in an out ten times that single day and it would be counted in the total logon value.

Internal Logons – Users who signed in on the internal network or through a VPN connection.  

Unfortunately a single user can sign in multiple times and it counts in this value. 

External Logons – Users who signed in from a remote connection (not VPN or internal network).  

Unfortunately a single user can sign in multiple times and it counts in this value. 

Unique logon users – Pertains to unique individual people that logged into the environment.  They don’t have to necessarily do anything once logged in. 

Unique active users - Pertains to unique individual people that logged into the environment and performed an action stated above such as IM, Conference, or Call.  

What is considered Concurrent?

Here is an example of something that I run into all the time when working through the design of the Front End servers and Edge servers.   Concurrent usage is how many users will be performing a task or service for that particular role at the same time.   The edge server can handle up to 15,000 remote SIP connections concurrently.  This mean that 15,000 users can be remote and the max supported connections for SIP that can be handled is 15,000 concurrent connections per Edge server. In my opinion that is a lot of connections.  Then add to the equation most deployments today consist of two edge servers for resiliency.  So that means the edge pool if it consist of two edge servers can handle 30,000 concurrent SIP connections.  By definition here, these numbers are assuming that the 15,000 connections have the capability to perform an action so they are not talking about 15,000 connections just sitting idle but rather the assumption that those 15,000 connections that registered through the edge server are idle and ready to perform an action at any time; so they are considered her an active SIP connection.       

Understand the Question 

At the end of the day, you need to understand that question that is being asked and in what context is the person who is asking question speaking of. If all else fells you can use this article as a baseline to assist others with understanding the difference between Active and Concurrent.