What does "Maintenance window" for installing agent mean ?

Hi everyone

I'm trying to deploy backup system for my network, using old but valid license for Veritas System Recovery 16

I've installed management solution and managed to deploy test zone - it's working fine, but I used option "Once ASAP" for installing software. There is also option called "Once at next maintenance window" - and it should be much more useful, but how does in work ? I've set a policy using this option - and nothing happens, not on next client reboot, not on next windows update reboot - just nothing. 

My test client runs Windows 7 Pro, joined AD 2012r2

So - what does this "maintenance window" mean ?