If you are licensed for Office Pro Plus (OPP) in your Office 365 license, you are allowed to install Office Professional Plus on up to five simultaneous machines.
But what if you lose count and try to install/activate Office Professional Plus on a sixth machine?
In the installation/activation attempt you will receive a message showing the machines that are currently active for your license. The message will instruct you to disable the subscription on one of your active machines to proceed.
You can always do the above maintenance exercise yourself at any time you prefer, using a command called osaui.exe, found in C:\Program Files\Common Files\microsoft shared\OFFICE14
If you use the /R parameter with the osaui.exe command you'll be presented with the above list of your computers on which you've activated OPP and be able remove one or more of these from your account.