https://www.zabbix.com/documentation/2.0/manual/config/notifications/action/conditions currently has:

"""
Maintenance status | in, not in |
Specify a host in maintenance or not in maintenance.
in - host is in maintenance mode.
not in - host is not in maintenance mode.
"""

This leaves room for speculation, what's going to happen, when multiple hosts are involved, some of them being in maintenance and others not.

I found the behaviour to be the following:

in: If at least one of the hosts IS in maintenance, the condition matches
not in: If at least one of the hosts IS NOT in maintenance, the condition matches