When sending keys to a VT host, the putCommand method is used to send the keys.  When the putCommand returns (or if the update is completed in the case where the putCommand times-out), this property can be checked to see if the changes sent from the host are "significant".

 

How significant the change is, is somewhat arbitrary but the main goal of this property is to avoid expensive operations on each receive from the host, for example while sending single keystrokes that simply echo the character.