What is difference between Obsolete and Inactive Clients?

I had to understand the difference as it was asked by the management and every administrator should know it.

Obsolete Clients

Obsolete client s are those that have been replaced by new ones. This usually happens during refresh OS deployments where the hardware stays the same and thus the hardware id is the same but the SMS GUID changes because the OS has been reloaded or the GUID is regenerated for another reason but the hardware remains the same.

Reasons - 
1. hard disk swapping
2. Renaming machines
3. Reimage OS
4. Reinstalling SMS/SCCM agent on the machines without proper uninstall.  

Inactive Clients

Inactive client s are those that have not been discovered recently by the heartbeat discovery. The definition of recently is defined in the delete task as a number of days. Please note that obsolete client s are also marked inactive. 

Reasons-
1. Offline machines
2. Machines having DNS issue/No name resolution
3. Machines are in inventory stock

Note: While I was trying to figure out why the some of the machines come under no status or waiting state, the above difference has helped me a lot.
I am putting some scenario here-

I have 100% healthy sms clients in the company's infrastructure and perform the activities like deleting obsolete clients, removing AD stale objects on daily basis. Inspite of this, I used to get some machines in "no status" and "waiting" category.

The reasons, I figured out, were:
offline machines/no name resolution machines were in "waiting" category and machines which were in IT stock or were inactive for a period of time, listed under "no status" category.