To put the RTB (Ready-To-Broadcast) validation on a stack :
•In the Browser, go to the "Stack" category.
•Select the stack.
•Go to the "RTB" column and double-click on it.
The "space shuttle" icon 
is displayed which means that the stack has the RTB validation and then it is visible in the playout system.
NB : if a stack does not have the RTB validation, it will not be visible in the playout system.