Peer availability
By default, a non-Meraki peer configuration applies to all MX-Z appliances in your Dashboard Organization. Since it is not always desirable for every appliance you control to form tunnels to a particular non-Meraki peer, the Availability column allows you to control which appliances within your Organization will connect to each peer. This control is based on network tags, which are labels you can apply to your Dashboard networks.
When "All networks" is selected for a peer, all MX-Z appliances in the organization will connect to that peer. When a specific network tag or set of tags is selected, only networks that have one or more of the specified tags will connect to that peer.
More information on network tags can be found here.