What is Notify all users
Undechannels, there is a checkbox called "Notify All Users", when it is checked for the channel, the users will receive a notification when a message received on that channel.
The default behaviour is that the user has to open the channel once to receive the notification whenever a new message is received on the channel. If the user has never opened the channel, he/she will not receive the notification of any new messages that are received on the channel.
But this behaviour can be overridden by checking the "Notify All Users" checkbox when it is checked, the user will receive notification for the messages that are received on that channel even if they have never opened that channel.