Notifiers are a communication mechanism between two independent parts of the block diagram or between two or more VIs running on the same machine. Notifiers are similar to mailboxes for data, one process sends data to the mailbox and one or several other processes receive data from the mailbox. In this way, they are similar to local or global variables. When using a variable, however, a process that is waiting for information must cycle continuously to check the value of the variable. One benefit of notifiers is that a process that receives information completely stops execution while waiting, and starts only when new data becomes available. This reduces excess processing power being wasted on unneeded polling.
In LabVIEW, the Notifier Operations
functions can be found under the Data Communication » Synchronization
palette as shown in the picture below.