Unique? Non-Unique? What to choose?

[J_Saun]

I am trying to change the following network object:

10.188.172.0/24

to

10.188.172.0/23

I receive the following error/pop-up:

"This IPv4 address is unique and already used by one or more networks in the system. Would you like to define this network's IPv4 addresses as non-unique?"

What does that mean and what do I choose? I do already have another object that is 10.188.173.0/24. Is that the conflict?